Annotate framework assemblies as trimmable (#48428)

Annotate framework assemblies as trimmable (#48428)

* Annotate framework assemblies as trimmable

This will opt them into trimming with the behavior described at https://github.com/mono/linker/blob/main/docs/design/trimmed-assemblies.md#assemblymetadataistrimmable-true.

* Add a condition to let projects opt out

A project just needs to set <SetIsTrimmable>false</SetIsTrimmable>, and
it won't get the trimmable metadata.

I also got rid of the SkipFrameworkAssemblyMetadata check since I don't
think the IsTrimmable metadata should be tied to whether we are building
a framework assembly, and this property was unused as far as I could tell.