[RuntimeAsync] TODOs in asyncthunks about methodtables directly encoded in signatures.

[VSadov]

There are 3 TODOs in the shape like:

    // TODO: (async) Encoding potentially shared method tables in
    // signatures of tokens seems odd, but this hits assert
    // with the typical method table.
    typeSigBuilder.AppendPointer(md->GetMethodTable());
    typeSigBuilder.AppendData(1);

We need to follow up on this and remove the TODOs as a result of:

• confirming that what we have is correct. Perhaps add a comment why it is correct.
• fix the code and ideally add tests.

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