Annotate struct size fields in metadata

It is common in windows API to have a "sizeof" field in a struct (usually called `dwSize`) for versioning. I believe metadata can introduce a new `[SizeOf]` attribute or similar and annotate such fields with it. Generators using metadata can then account for that attribute and automatically assign right value to the annotated field. For instance, CsWin32 can do that in a friendly function overload

Begin with the metadata model and the generators that consume it, then determine how a proposed SizeOf annotation would be represented and surfaced to CsWin32. Done means metadata can mark struct size fields and generators can use that marker to assign the appropriate value in a friendly overload.

Written by the indexing model from the issue text.