Add index-only scan support to inet GiST opclass.

src/backend/utils/adt/network_gist.c

src/include/catalog/catversion.h

src/include/catalog/pg_amproc.h

src/include/catalog/pg_proc.h

src/include/utils/inet.h

src/test/regress/expected/inet.out

src/test/regress/sql/inet.sql