In https://gitlab.com/gitlab-org/gitaly/-/issues/4459, we introduced
Repository actor for Gitaly. This actor targets a particular repository
managed by Gitaly. It uses repository relative path as unique identity
for flipper_id. This commit adds the support to enable/disable a flag
for a repository to admin Feature API.

Issue: https://gitlab.com/gitlab-org/gitaly/-/issues/4549


Changelog: added
EE: true
Signed-off-by: Quang-Minh Nguyen <qmnguyen@gitlab.com>