Add support for the new list-owner rest API
Add support for the new list-owner rest API

The backend is going to return an object instead of an array.
See the following change for the details:
https://gerrit-review.googlesource.com/c/plugins/code-owners/+/293806

Change-Id: I61b5dba2cc725779c9c5b32c4f089727fde87c8f

GerritCodeReview/plugins_code-owners,GerritCodeReview/plugins_code-owners,GerritCodeReview/plugins_code-owners,GerritCodeReview/plugins_code-owners