feat(events): add events management capabilities

This change introduces some new plugins which provide the basics
for managing events inside of backstage.
Hereby, it offers extension points to add event publishers and subscribers
as well as to exchange the event broker implementation.

- `@backstage/plugin-events-backend`:
  backend for the events management which connects all parts
  and provides a simple in-memory event broker
- `@backstage/plugin-events-node`:
  interfaces and API for `@backstage/plugin-events-backend`
- `@backstage/plugin-events-test-utils`:
  test utilities like implementations useful for writing tests at modules

All plugins support the new backend-plugin-api.

Relates-to: #11082
Signed-off-by: Patrick Jungermann <Patrick.Jungermann@gmail.com>

plugins/events-backend/README.md

@@ -0,0 +1,124 @@
+# events-backend
+
+Welcome to the events-backend backend plugin!
+
+This plugin provides the wiring of all extension points
+for managing events as defined by [plugin-events-node](../events-node)
+including backend plugin `EventsPlugin` and `EventsBackend`.
+
+Additionally, it uses a simple in-memory implementation for
+the `EventBroker` by default which you can replace with a more sophisticated
+implementation of your choice as you need (e.g., via module).
+
+Some of these (non-exhaustive) may provide added persistence,
+or use external systems like AWS EventBridge, AWS SNS, Kafka, etc.
+
+## Installation
+
+```bash
+# From your Backstage root directory
+yarn add --cwd packages/backend @backstage/plugin-events-backend
+```
+
+Add a file [`packages/backend/src/plugins/events.ts`](../../packages/backend/src/plugins/events.ts)
+to your Backstage project.
+
+There, you can add all publishers, subscribers, etc. you want.
+
+Additionally, add the events plugin to your backend.
+
+```diff
+// packages/backend/src/index.ts
+// [...]
++import events from './plugins/events';
+// [...]
++  const eventsEnv = useHotMemoize(module, () => createEnv('events'));
+// [...]
++  apiRouter.use('/events', await events(eventsEnv, []));
+// [...]
+```
+
+### With Event-based Entity Providers
+
+In case you use event-based `EntityProviders`,
+you may need something like the following:
+
+```diff
+// packages/backend/src/index.ts
+-  apiRouter.use('/events', await events(eventsEnv, []));
++  apiRouter.use('/events', await events(eventsEnv, eventBasedEntityProviders));
+```
+
+as well as a file
+[`packages/backend/src/plugins/catalogEventBasedProviders.ts`](../../packages/backend/src/plugins/catalogEventBasedProviders.ts)
+which contains event-based entity providers.
+
+In case you don't have this dependency added yet:
+
+```bash
+# From your Backstage root directory
+yarn add --cwd packages/backend @backstage/plugin-events-backend
+```
+
+```diff
+// packages/backend/src/plugins/catalog.ts
+ import { CatalogBuilder } from '@backstage/plugin-catalog-backend';
++import { EntityProvider } from '@backstage/plugin-catalog-node';
+ import { ScaffolderEntitiesProcessor } from '@backstage/plugin-scaffolder-backend';
+ import { Router } from 'express';
+ import { PluginEnvironment } from '../types';
+
+ export default async function createPlugin(
+   env: PluginEnvironment,
++  providers?: Array<EntityProvider>,
+ ): Promise<Router> {
+   const builder = await CatalogBuilder.create(env);
+   builder.addProcessor(new ScaffolderEntitiesProcessor());
++  builder.addEntityProvider(providers ?? []);
+   const { processingEngine, router } = await builder.build();
+   await processingEngine.start();
+   return router;
+ }
+```
+
+## Use Cases
+
+### Custom Event Broker
+
+Example using the `EventsBackend`:
+
+```ts
+new EventsBackend(env.logger)
+  .setEventBroker(yourEventBroker)
+  // [...]
+  .start();
+```
+
+Example using a module:
+
+```ts
+import { eventsExtensionPoint } from '@backstage/plugin-events-node';
+
+// [...]
+
+export const yourModuleEventsModule = createBackendModule({
+  pluginId: 'events',
+  moduleId: 'yourModule',
+  register(env) {
+    // [...]
+    env.registerInit({
+      deps: {
+        // [...]
+        events: eventsExtensionPoint,
+        // [...]
+      },
+      async init({ /* ... */ events /*, ... */ }) {
+        // [...]
+        const yourEventBroker = new YourEventBroker();
+        // [...]
+        events.setEventBroker(yourEventBroker);
+      },
+    });
+  },
+});
+```