Support running optional lite and full source-build pipelines in SDK PR Checks

[ellahathaway]

In SDK PR checks, we have the following VMR pipelines:

• sdk-unified-build
• sdk-unified-build-full
• sdk-source-build

Both sdk-unified-build and sdk-soure-build run as required pipeline checks for SDK PRs, while sdk-unified-build-full is an optional check that can be triggered via the azp run command.

Source-build should consider following the approach used by the unified build pipelines. Specifically, we should consider adding optional public pipelines called sdk-source-build-lite and sdk-source-build-full that run either the source-build lite legs or the full source-build legs, respectively.

[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.

[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.

[MichaelSimons]

[Triage] This will require creating a new pipeline(s). Once SB is migrated into the VMR, this will be trivial to support.