Add source build leg to the official unified build pipeline to produce intermediates and "release" artifacts

[MichaelSimons]

With the unified build project, the official builds need to produce the following assets. This implies a single source-build leg will need to be added

1. Source build intermediate packages for consumption in the product repo source build legs. Related to Add support for producing intermediate packages from the product source build #4754
2. Microsoft's source built artifacts necessary for bootstrapping source build. This will be "released" as part of the source build release.
3. Any source built sdks necessary for testing the next release. source-build CI validates the next version can be built with the previous source built sdk.

[dotnet-issue-labeler]

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[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.