Add SupportRequestInlineInFrom planner support request.

Add SupportRequestInlineInFrom planner support request.

This request allows a support function to replace a function call
appearing in FROM (typically a set-returning function) with an
equivalent SELECT subquery.  The subquery will then be subject
to the planner's usual optimizations, potentially allowing a much
better plan to be generated.  While the planner has long done this
automatically for simple SQL-language functions, it's now possible
for extensions to do it for functions outside that group.
Notably, this could be useful for functions that are presently
implemented in PL/pgSQL and work by generating and then EXECUTE'ing
a SQL query.

Author: Paul A Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/09de6afa-c33d-4d94-a5cb-afc6cea0d2bb@illuminatedcomputing.com