Consider supporting `AsyncMethodBuilder` 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

[dotnet-policy-service]

Tagging subscribers to this area: @steveisok, @dotnet/area-system-reflection-emit
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.