rgw: add rgw_owner variant with json encoding

rgw: add rgw_owner variant with json encoding

existing buckets and objects use `struct rgw_user` for the owner. with
the addition of accounts, we need to be able to represent ownership by
the account rather than its users

add a `rgw_owner` variant that preserves the existing json encoding of
`rgw_user` while adding a new representation for account ids

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 87a74f8603bdeccdea3fdfa7b4e0fd344fba8aae)