Consider supporting `[AsyncMethodBuilder(typeof(PoolingAsyncValueTaskMethodBuilder))]` in runtime-async

[BrennanConroy]

Right now methods marked with [AsyncMethodBuilder(typeof(PoolingAsyncValueTaskMethodBuilder))] are showing a regression from pre-runtime-async due to allocations. This issue is opened to consider if there should be support for AsyncMethodBuilder or a similar mechanism in runtime-async.

For now, runtime-async is being disabled for methods with [AsyncMethodBuilder(...)]. See #128943

@jakobbotsch

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[JulieLeeMSFT]

@jakobbotsch, putting this to future. Please feel free to pull in.

[jakobbotsch]

I don't think we can support the completely general AsyncMethodBuilder, but we can introduce something like [PoolContinuations] that implements the same kind of poling as you would get using PoolingAsyncValueTaskMethodBuilder.