rgw: Swift API. Dump container's custom metadata.

rgw: Swift API. Dump container's custom metadata.

Fixes: #10665
Backport: hammer, firefly
Reported-by: Ahmad Faheem <ahmad.faheem@ril.com>
Signed-off-by: Dmytro Iurchenko <diurchenko@mirantis.com>
(cherry picked from commit 2f8d31e9b1c4b09506bd1b0dad635c6e820783b2)

Conflicts:
	src/rgw/rgw_rest_swift.cc
        The first hunk conflicts because X-Storage-Policy was added
	after firefly. The second hunk conflicts because the type of the
	s->object data member changed after firefly but it is irrelevant
	because the patch does not need to access s->object anymore.