[RuntimeAsync] Implement struct async thunks.

[VSadov]

Re: https://github.com/dotnet/runtimelab/pull/2951/files#r1960790542

There is an assert:

       // Struct async thunks not yet implemented
       _ASSERTE(!this->GetMethodTable()->IsValueType());

It can be hit by Async2Struct testcase by changing the void Async() entry point to be an ordinary async method.

      private static async Task Async()    // not async2

Some of the scenarios in the test may become irrelevant since it will be C# responsibility to capture by value.

However the general case of an async struct method still needs to be handled, so this needs a follow up.