Implement a change in IL API to use RuntimeHelpers.Await<T>(Task<T>) and similar helpers.

[VSadov]

This is actual implementation of what is proposed in dotnet/runtime#110420
and prototyped in #2941

NOTE: This PR has only the VM/runtime parts.
The JIT optimization to optimize Await(RuntimeAsyncMethod), which is probably the harder part of the proposal, is not included here.

The code works and passes tests, but the optimization is important to avoid redundant thunking when one runtime-async method awaits another as such scenario could be common and can be lowered into a more efficient direct call.

[VSadov]

@jakobbotsch the change stresses calling via thunks and possibly introduced some scenarios that tests did not cover before. Remarkably, nearly everything works fine!! However, here I saw an assert and turned off one scenario.
Not sure if this is something wrong with IL or something on the JIT side.
(the other case is with thunks for async methods in structs).

[jakobbotsch]

The JIT optimization to optimize Await(RuntimeAsyncMethod), which is probably the harder part of the proposal, is not included here.

It would be nice to start on this work to see how it would look before we make the switch. Note that most of the work will be VM work -- teaching getCallInfo implementations to deal with the fact that it now may need to describe a call to the async variant of a call described by a token.