Split out pg_operator.h function declarations to new file pg_operator_fn.h.

Split out pg_operator.h function declarations to new file pg_operator_fn.h.

Commit a2e35b53c39b2a27 added an #include of catalog/objectaddress.h to
pg_operator.h, making it impossible for client-side code to #include
pg_operator.h.  It's not entirely clear whether any client-side code needs
to include pg_operator.h, but it seems prudent to assume that there is some
such code somewhere.  Therefore, split off the function definitions into a
new file pg_operator_fn.h, similarly to what we've done for some other
catalog header files.

Back-patch of part of commit 0dab5ef39b3d9d86.