Enable ETW/EventSource logging of task IDs for boxed state machines (#27115) (#27217)

Enable ETW/EventSource logging of task IDs for boxed state machines (#27115) (#27217)

* Wrap MoveNext Action for TPL event tracing

Wraps the MoveNext action of the AsyncStateMachineBox in a
continuation wrapper when async causality tracing is on so that the
TPL event source can find the task that is associated with a
continuation. Does not wrap otherwise.

* Clarifying comment

* removing trailing whitespace

* code review feedback.

Makes AsyncMethodBuilderCore.TryFindContinuationTask check to
whether the target of a continuation is itself a task, as a fall-back
to checking to see if the continuation is a ContinuationWrapper

* Got rid of unnecessary null checks