Replace simple constant pg_am.amcanreturn with an AM support function.

The need for this was debated when we put in the index-only-scan feature,
but at the time we had no near-term expectation of having AMs that could
support such scans for only some indexes; so we kept it simple.  However,
the SP-GiST AM forces the issue, so let's fix it.

This patch only installs the new API; no behavior actually changes.

src/include/utils/rel.h

@@ -64,6 +64,7 @@ typedef struct RelationAmInfo
 	FmgrInfo	ambuildempty;
 	FmgrInfo	ambulkdelete;
 	FmgrInfo	amvacuumcleanup;
+	FmgrInfo	amcanreturn;
 	FmgrInfo	amcostestimate;
 	FmgrInfo	amoptions;
 } RelationAmInfo;