Designate DNS - List project limits. No project ID is included in response

### Scenario ###
Activate API to get “project limits” for all projects using Admin client and “x-auth-all-projects: True”

### Actual result ###
Received API response is dictionary, there is no way to know whom (project) those received values are associated with.

2021-06-24 11:52:23.414 4116243 INFO tempest.lib.common.rest_client [req-82a52407-6f02-42de-8139-6846920d8522 ] Request (DesignateLimit:test_list_designate_limits_all_projects): 200 GET http://10.35.64.8/dns/v2/limits 0.162s
2021-06-24 11:52:23.414 4116243 DEBUG tempest.lib.common.rest_client [req-82a52407-6f02-42de-8139-6846920d8522 ] Request - Headers: {'x-auth-all-projects': True, 'X-Auth-Token': '<omitted>'}
        Body: None
    Response - Headers: {'date': 'Thu, 24 Jun 2021 08:52:23 GMT', 'server': 'Apache/2.4.41 (Ubuntu)', 'content-length': '203', 'x-openstack-request-id': 'req-82a52407-6f02-42de-8139-6846920d8522', 'connection': 'close', 'content-type': 'application/json', 'status': '200', 'content-location': 'http://10.35.64.8/dns/v2/limits'}
        Body: b'{"max_zones": 10, "max_zone_recordsets": 500, "max_zone_records": 500, "max_recordset_records": 20, "min_ttl": null, "max_zone_name_length": 255, "max_recordset_name_length": 255, "max_page_limit": 1000}' _log_request_full /opt/stack/tempest/tempest/lib/common/rest_client.py:450

### Expected result ####
Basing on Documentation we have:
https://docs.openstack.org/api-ref/dns/?expanded=set-floatingip-s-ptr-record-detail,unset-floatingip-s-ptr-record-detail,get-project-limits-detail#get-project-limits we would probably had to respond with some other data structure (maybe list of dictionaries) where project_id is included.

Note: it's possible to use: https://review.opendev.org/c/openstack/designate-tempest-plugin/+/797678 (patchset #2) for reproducing.