Investigate using source generators to simplify handling return buffer semantics when overriding C++ classes from C#

[PathogenDavid]

We could use source generators to automatically generate the vtable stuff for C# overrides of C++ classes to avoid having the C# consumer from worrying about the thisptr/retbuf semantics without sacrificing performance or generating a bunch of different vtables.

[PathogenDavid]

Related: https://github.com/InfectedLibraries/Biohazrd/issues/89