New header varatt.h split off from postgres.h

This new header contains all the variable-length data types support
(TOAST support) from postgres.h, which isn't needed by large parts of
the backend code.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/ddcce239-0f29-6e62-4b47-1f8ca742addf%40enterprisedb.com

src/backend/libpq/be-fsstubs.c

@@ -53,6 +53,7 @@
 #include "utils/builtins.h"
 #include "utils/memutils.h"
 #include "utils/snapmgr.h"
+#include "varatt.h"
 
 /* define this to enable debug logging */
 /* #define FSDB 1 */

src/backend/libpq/pqformat.c

@@ -77,6 +77,7 @@
 #include "libpq/pqformat.h"
 #include "mb/pg_wchar.h"
 #include "port/pg_bswap.h"
+#include "varatt.h"
 
 
 /* --------------------------------