rgw: add 'GET /admin/realm?list' api to list realms

rgw: add 'GET /admin/realm?list' api to list realms

returns the same json format as 'radosgw-admin realm list', where
'default_info' is the id of the default realm, and 'realms' is the list
of realm names

the user must authenticate and have the 'zone' read capability

~/ceph/build $ curl http://localhost:8000/admin/realm?list
{"default_info":"52681550-8531-407c-b734-b588e5221883","realms":["bar","foo","dev"]}

~/ceph/build $ bin/radosgw-admin realm list
{
    "default_info": "52681550-8531-407c-b734-b588e5221883",
    "realms": [
        "bar",
        "foo",
        "dev"
    ]
}

Fixes: https://tracker.ceph.com/issues/39626
Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 856d078cf478893d1081bda40707ac189c212739)