Generate metadata in manifest.

[mitchdenny]

Fixes: #1959

This PR adds infrastructure to support emitting a "metadata": property for resources in the manifest. The usage is as follows:

var builder = DistributedApplication.CreateBuilder(args);
builder.AddProject<Projects.MyApp>("myapp")
       .WithMetadata("azd-identity", "guid-for-pre-existing-identity");

The manifest would look like this:

{
  "resources": {
    "myapp": {
      // some properties omitted for brevity.
      "type": "project.v0",
      "metadata": {
        "azd-identity": "guid-for-pre-existing-identity"
     }, 
   }
  }
}

Metadata properties use the JSON serializer on the value field so they can be simple types such as strings, booleans, and integers, but they can also be complex objects.

Microsoft Reviewers: Open in CodeFlow

[davidfowl]

string?

[mitchdenny]

On hold for now but based on latest offline chat we might still allow object serialization.

[mitchdenny]

@davidfowl merge conflict resolved. This metadata PR was ready to go in unless you can spot some issues with the current implementation.

[davidfowl]

RIP AOT 😄

[mitchdenny]

We can probably inject a serializer via DI here but TBH it don't think anyone would be doing AOT on their app host, given the product we are shipping for GA there is no reason to wait for the native build when you just want to launch the app to either debug locally or produce a manifest.

[davidfowl]

This is going to end up killed our perf when we do testing 😄.

[mitchdenny]

I can think of a many over engineered solutions which I'll be happy to implement once we get some good data on where our perf pain is.