Add support for custom User Assigned Identities against Aspire Projects

[rudiv]

See dotnet/aspire#3339 for more information.

This change adds support to azd to consume a new userAssignedIdentities property in the Aspire manifest.

An example being:

"userAssignedIdentities": [
  {
    "clientId": "00000000-0000-0000-0000-000000000000",
    "resourceId": "/subscriptions/\u003Csubscription_id\u003E/resourcegroups/my-rg/providers/Microsoft.ManagedIdentity/userAssignedIdentities/my-user",
    "env": "TEST"
  },
  {
    "clientId": "{identities.outputs.clientId}",
    "resourceId": "{identities.outputs.resourceId}",
    "env": "TEST2"
  }
]

The change adds support for:

1. Detection of template strings within this new property
2. The generation of environment variables to support the Client IDs
3. The addition of the userAssignedIdentities in the container's yaml

[microsoft-github-policy-service]

Thank you for your contribution rudiv! We will review the pull request and get back to you soon.

[ellismg]

I'm going to close this for now - based on the discussion in dotnet/aspire#3339 we do want to support this scenario long term, but we don't think this is the right way to do it (and it likely is something we'll want to do post GA once we move the logic for created the IAC for Container Apps into the Aspire deployment package and outside of azd.