Fix rpc call from name based Callables

[AThousandShips]

Makes rpc calls work for the Callable(obj, method) form Callables, works like the GDScriptRPCCallable call (which also does no special treatment for the error return, I'll leave it as such and leave any improvements or changes to that side to future improvements to both)

• Fixes: Callable.rpc() on RPC function throws an error "Invalid call. Nonexistent function 'rpc' in base 'Callable'. #87841

[KoBeWi]

This fixes the error, but not sure how to confirm whether the method is actually called.
The code looks fine though.

[Faless]

This looks great to me!

I remember @reduz wanted this to be implemented at the language level using custom callables, but I think it make sense to also have the option of using Callable(obj, method) as an alternative if the language doesn't implement it.

[akien-mga]

Thanks!

[AThousandShips]

Thank you!