Adding Azure DevOps backend plugin

Signed-off-by: Andre Wanlin <awanlin@rapidrtc.com>

plugins/azure-devops-backend/README.md

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+# Azure DevOps Backend
+
+Simple plugin that proxies requests to the [Azure DevOps](https://docs.microsoft.com/en-us/rest/api/azure/devops/?view=azure-devops-rest-6.1) API.
+
+## Setup
+
+The following values are read from the configuration file:
+
+```yaml
+azureDevOps:
+  host: dev.azure.com
+  token: ${AZURE_TOKEN}
+  organization: my-company
+  top: 50
+```
+
+Configuration Details:
+
+- `host` and `token` can be the same as the ones used for the `integration` section
+- `AZURE_TOKEN` environment variable must be set to a [Personal Access Token](https://docs.microsoft.com/en-us/azure/devops/organizations/accounts/use-personal-access-tokens-to-authenticate?view=azure-devops&tabs=preview-page) with read access to both Code and Build
+- `organization` is your Azure DevOps Organization name or for Azure DevOps Server (on-premise) this will be your Collection name
+- `top` sets the max number of items to return - currently only applies to builds
+
+## Links
+
+- [Frontend part of the plugin](https://github.com/backstage/backstage/tree/master/plugins/azure-devops)
+- [The Backstage homepage](https://backstage.io)