rgw: send user manifest header field

Fixes: #8170
Backport: firefly
If user manifest header exists (swift) send it as part of the object
header data.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 5cc5686039a882ad345681133c9c5a4a2c2fd86b)