Make it possible to use GenFacades' GenFacadesForceZeroVersionSeeds feature without being a partial facade assembly
(IsPartialFacadeAssembly).

Also add a switch to disable the automated addition of the generated GenFacades compile item.

Nit: This reduces evaluation time slightly as the ClearAssemblyReferenceVersions logic doesn't need to be evaluated for projects that don't use its functionality but that use IsPartialFacadeAssembly. In dotnet/runtime, that's the majority of projects.

To double check:

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