rgw/admin: add an interface around the HTTP Client
rgw/admin: add an interface around the HTTP Client

We now have a new interface `HTTPClient` which helps us doing various
operations such as mutating and mocking the HTTP Client.

Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

ceph/go-ceph,ceph/go-ceph,ceph/go-ceph