Implement ElasticSearch search engine

* Adding indexing, searching and default translator for ElasticSearch engine
* Modifying default backend to use ES if it is enabled
* Adding configuration schema to configure ElasticSearch 3 different ways
   * Elastic.co hosted solution
   * AWS hosted ElasticSearch Service
   * Custom, using standard ElasticSearch URL and auth info
* Add and modify some of the documentation regarding search

Signed-off-by: Jussi Hallila <jussi@hallila.com>

docs/features/search/getting-started.md

@@ -252,13 +252,9 @@ an example:
 Backstage Search isn't a search engine itself, rather, it provides an interface
 between your Backstage instance and a
 [Search Engine](./concepts.md#search-engines) of your choice. Currently, we only
-support one, an in-memory search Engine called Lunr. It can be instantiated like
-this:
-
-```typescript
-const searchEngine = new LunrSearchEngine({ logger });
-const indexBuilder = new IndexBuilder({ logger, searchEngine });
-```
+support two engines, an in-memory search Engine called Lunr and ElasticSearch.
+See [Search Engines](./search-engines.md) documentation for more information how
+to configure these in your Backstage instance.
 
 Backstage Search can be used to power search of anything! Plugins like the
 Catalog offer default [collators](./concepts.md#collators) (e.g.

docs/features/search/search-engines.md

@@ -0,0 +1,141 @@
+---
+id: search-engines
+title: Search Engines
+description: Choosing and configuring your search engine for Backstage
+---
+
+# Search Engines
+
+Backstage supports 2 search engines by default, an in-memory engine called Lunr
+and ElasticSearch. You can configure your own search engines by implementing the
+provided interface as mentioned in the
+[search backend documentation.](./getting-started.md#Backend)
+
+Provided search engine implementations have their own way of constructing
+queries, which may be something you want to modify. Alterations to the querying
+logic of a search engine can be made by providing your own implementation of a
+QueryTranslator interface. This modification can be done without touching
+provided search engines by using the exposed setter to set the modified query
+translator into the instance.
+
+```
+const searchEngine = new LunrSearchEngine({ logger });
+searchEngine.setTranslator(new MyNewAndBetterQueryTranslator());
+```
+
+## Lunr
+
+Lunr search engine is enabled by default for your backstage instance if you have
+not done additional changes to the scaffolded app.
+
+Lunr can be instantiated like this:
+
+```typescript
+// app/backend/src/plugins/search.ts
+const searchEngine = new LunrSearchEngine({ logger });
+const indexBuilder = new IndexBuilder({ logger, searchEngine });
+```
+
+## ElasticSearch
+
+Backstage supports ElasticSearch search engine connections, indexing and
+querying out of the box. Available configuration options enable usage of either
+AWS or Elastic.co hosted solutions, or a custom self-hosted solution.
+
+Similarly to Lunr above, ElasticSearch can be set up like this:
+
+```typescript
+// app/backend/src/plugins/search.ts
+const searchEngine = await ElasticSearchSearchEngine.initialize({
+  logger,
+  config,
+});
+const indexBuilder = new IndexBuilder({ logger, searchEngine });
+```
+
+For the engine to be available, your backend package needs a dependency into
+package `@backstage/plugin-search-backend-module-elasticsearch`.
+
+ElasticSearch needs some additional configuration before it is ready to use
+within your instance. The configuration options are documented in the
+[configuration schema definition file.](https://github.com/backstage/backstage/blob/master/plugins/search-backend-module-elasticsearch/config.d.ts)
+
+## Example configurations
+
+### AWS
+
+Using AWS hosted ElasticSearch the only configuration option needed is the URL
+to the ElasticSearch service. The implementation assumes that environment
+variables for AWS access key id and secret access key are defined in accordance
+to the
+[default AWS credential chain.](https://docs.aws.amazon.com/sdk-for-javascript/v2/developer-guide/setting-credentials-node.html).
+
+```yaml
+search:
+  elasticSearch:
+    provider: aws
+    node: https://my-backstage-search-asdfqwerty.eu-west-1.es.amazonaws.com
+```
+
+### Elastic.co
+
+Elastic Cloud hosted ElasticSearch uses a Cloud ID to determine the instance of
+hosted ElasticSearch to connect to. Additionally, username and password needs to
+be provided either directly or using environment variables like defined in
+[Backstage documentation.](https://backstage.io/docs/conf/writing#includes-and-dynamic-data)
+
+```yaml
+search:
+  elasticSearch:
+    provider: elastic
+    cloudId: backstage-elastic:asdfqwertyasdfqwertyasdfqwertyasdfqwerty==
+    auth:
+      username: elastic
+      password: changeme
+```
+
+### Others
+
+Other ElasticSearch instances can be connected to by using standard
+ElasticSearch authentication methods and exposed URL, provided that the cluster
+supports that. The configuration options needed are the URL to the node and
+authentication information. Authentication can be handled by either providing
+username/password or an API key or a bearer token. In case both
+username/password combination and one of the tokens are provided, token takes
+precedence. For more information how to create an API key, see
+[Elastic documentation on API keys](https://www.elastic.co/guide/en/elasticsearch/reference/current/security-api-create-api-key.html),
+and how to create a bearer token see
+[Elastic documentation on tokens.](https://www.elastic.co/guide/en/elasticsearch/reference/current/security-api-create-service-token.html)
+
+#### Configuration examples
+
+##### With username and password
+
+```yaml
+search:
+  elasticSearch:
+    node: http://localhost:9200
+    auth:
+      username: elastic
+      password: changeme
+```
+
+##### With bearer token
+
+```yaml
+search:
+  elasticSearch:
+    node: http://localhost:9200
+    auth:
+      bearer: token
+```
+
+##### With API key
+
+```yaml
+search:
+  elasticSearch:
+    node: http://localhost:9200
+    auth:
+      apiKey: base64EncodedKey
+```