[RuntimeAsync] TODO in runtimehandles.cpp

[VSadov]

//  TODO: (async) the filtering may not cover all scenarios. Review and add tests.

The idea is that the async thunks are not exposed to reflection. Async helpers like Await, which are real methods in metadata might be exposed, but should not be invokable.

It is likely that we already have all end covered.
Just need to make sure that all ways to get an async thunk via reflection are blocked. Add an assert in some choke point if it is not already there.

Also need make sure that Await cannot be executed via reflection. Add a test if not already there. (I think we have tests).

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