[ffigen] Include `API_AVAILABLE` annotations in ObjC code

[liamappelbe]

Our generated ObjC code should use API_AVAILABLE annotations where appropriate. For example, block trampolines should be annotated with API_AVAILABLE corresponding to the types in the block's signature.

The thing that makes this non-trivial is that a block can have arbitrarily many types in its signature, so we'll need to union the availability annotations from all of them. Best way to do this would be to add a method on Type that calculates the ApiAvailability of the type.

Related to #2904

[sagar-h007]

Hi @liamappelbe , I’d like to work on this.
My plan is to add availability aggregation logic to the Type model so it can compute combined ApiAvailability from all nested signature types, then update ObjC generation to emit the correct API_AVAILABLE annotations. I’ll start by tracing where availability metadata is currently parsed and propagated, implement the union logic, and add tests for block trampolines and mixed-availability signatures.