[FEATURE REQ] Managed Identity support for Backends

[Jeroen-VdB]

Please describe the feature.

In the Azure Portal I configured my backend credentials using a Managed Identity like this:

When I extract the artifacts, the managed identity configuration is not included in the backendInformation.json file:

{
  "properties": {
    "credentials": {
      "header": {},
      "query": {}
    },
    "protocol": "http",
    "tls": {
      "validateCertificateChain": true,
      "validateCertificateName": true
    },
    "url": "https://test.com"
  }
}

Would credentials be the place to store the managed identity properties?
For example:

{
  "properties": {
    "credentials": {
      "managedIdentity": {
         "clientId":"my-managed-identity-client-id",
         "resourceId":"api://example-scope", // I used the name from the portal as a property name, which makes sense for Azure resource backends (e.g. Service Bus, Key Vault, etc), but from an app reg OAuth perspective the name "scope" would be better suited in my opinion
      },
    },
    "protocol": "http",
    "tls": {
      "validateCertificateChain": true,
      "validateCertificateName": true
    },
    "url": "https://test.com"
  }
}

I am open to contributing it myself if the maintainers find it a good idea and we can agree on the implementation details before I start developing.

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