pg_dump: Add --sequence-data.

This new option instructs pg_dump to dump sequence data when the
--no-data, --schema-only, or --statistics-only option is specified.
This was originally considered for commit a7e5457db8, but it was
left out at that time because there was no known use-case.  A
follow-up commit will use this to optimize pg_upgrade's file
transfer step.

Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/Zyvop-LxLXBLrZil%40nathan

src/test/modules/test_pg_dump/t/001_base.pl

@@ -48,7 +48,7 @@ my %pgdump_runs = (
 		dump_cmd => [
 			'pg_dump', '--no-sync',
 			'--file' => "$tempdir/binary_upgrade.sql",
-			'--schema-only', '--binary-upgrade',
+			'--schema-only', '--sequence-data', '--binary-upgrade',
 			'--dbname' => 'postgres',
 		],
 	},