Add red-black tree missing comparison searches

Hello hackers,

Currently the red-black tree implementation only has an equality search.
Other extensions might need other comparison searches, like less-or-equal
or greater-or-equal. For example OrioleDB defines a greater-or-equal search
on its postgres fork:

https://github.com/orioledb/postgres/blob/4c18ae94c20e3e95c374b9947f1ace7d1d6497a1/src/backend/lib/rbtree.c#L164-L186

So I thought this might be valuable to have in core. I've added
less-or-equal and greater-or-equal searches functions plus tests in
the test_rbtree module. I can add the remaining less/great searches if this
is deemed worth it.

Also I refactored the sentinel used in the rbtree.c to use C99 designators.

Thanks in advance for any feedback!

--
Steve Chavez
Engineering at https://supabase.com/