Factor out plpgsql's management of its function cache.

SQL-language functions need precisely this same functionality to
manage a long-lived cache of functions.  Rather than duplicating
or reinventing that code, let's split it out into a new module
funccache.c so that it is available for any language that wants
to use it.

This is mostly an exercise in moving and renaming code, and should
not change any behavior.  I have added one feature that plpgsql
doesn't use but SQL functions will need: the cache lookup key
can include the output tuple descriptor when the function
returns composite.

Author: Tom Lane <[email protected]>
Discussion: https://postgr.es/m/8216639.NyiUUSuA9g@aivenlaptop

Author: tglsfdc

src/backend/utils/cache/Makefile

@@ -16,6 +16,7 @@ OBJS = \
 	attoptcache.o \
 	catcache.o \
 	evtcache.o \
+	funccache.o \
 	inval.o \
 	lsyscache.o \
 	partcache.o \