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/access/common/toast_compression.c

@@ -22,6 +22,7 @@
 #include "common/pg_lzcompress.h"
 #include "fmgr.h"
 #include "utils/builtins.h"
+#include "varatt.h"
 
 /* GUC */
 int			default_toast_compression = TOAST_PGLZ_COMPRESSION;

src/backend/access/hash/hashfunc.c

@@ -32,6 +32,7 @@
 #include "utils/builtins.h"
 #include "utils/float.h"
 #include "utils/pg_locale.h"
+#include "varatt.h"
 
 /*
  * Datatype-specific hash functions.

src/backend/access/spgist/spgtextproc.c

@@ -46,6 +46,7 @@
 #include "utils/datum.h"
 #include "utils/pg_locale.h"
 #include "utils/varlena.h"
+#include "varatt.h"
 
 
 /*

src/backend/access/table/toast_helper.c

@@ -19,6 +19,7 @@
 #include "access/toast_helper.h"
 #include "access/toast_internals.h"
 #include "catalog/pg_type_d.h"
+#include "varatt.h"
 
 
 /*