Use pipeline variable to reference MAR Status API resource/scope

[lbussell]

Currently we hardcode the resource scope for the MAR status API:

docker-tools/src/Microsoft.DotNet.ImageBuilder/src/McrStatusClient.cs

Line 20 in 7b8f032

private const string McrStatusResource = "api://c00053c3-a979-4ee6-b94e-941881e62d8e";

This is very inflexible and requires changes to our source code if we need to change where this points to. Instead, we should accept a scope via the CLI and pass it in via an environment variable so that we can change it as needed without rebuilding ImageBuilder. We don't intend to frequently access this API on dev machines, so this should be an acceptable solution.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[mthalman]

It would probably make sense to define this in our Dotnet-Docker-Common variable group.

[lbussell]

There is a WIP branch for this here - https://github.com/lbussell/docker-tools/tree/issue-1288