Add a new API rb_profile_thread_frames(), which is essentialy a
per-thread version of rb_profile_frames().

While the original rb_profile_frames() always returns results about the
current active thread obtained by GET_EC(), this new API takes a Thread
to be profiled as an argument.

This should come in handy when profiling I/O-bound programs such as
webapps, since this new API allows us to learn about Threads performing
I/O (which do not have the GVL).

Profiling worker threads (such as Sidekiq workers) may be another
application.

Implements [Feature #10602]

Co-authored-by: Mike Perham <mike@perham.net>