Implement the spec to define projects as part of a join vertical in a unified build

[jkoritzinsky]

Implement the spec at https://github.com/dotnet/arcade/blob/main/Documentation/UnifiedBuild/Unified-Build-Join-Point.md#declare-projects-to-build-in-a-join-vertical

Skip projects with a different DotNetBuildPass value, don't include solution files by default in later build passes, and don't error for no projects in later build passes.

Contributes to https://github.com/dotnet/source-build/issues/3739

To double check:

• [ ] The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md