feat(datastore): add index to speed up lookups of manifests by tag

See merge request gitlab-org/container-registry!1224



Merged-by: Hayley Swimelar <hswimelar@gitlab.com>
Approved-by: Suleimi Ahmed <sahmed@gitlab.com>
Approved-by: Simon Tomlinson <stomlinson@gitlab.com>
Approved-by: Hayley Swimelar <hswimelar@gitlab.com>
Co-authored-by: João Pereira <jpereira@gitlab.com>

# [3.68.0](gitlab-org/container-registry@v3.67.1-gitlab...v3.68.0-gitlab) (2023-03-01)

### Features

* **datastore:** add index to speed up lookups of manifests by tag ([069a7051](gitlab-org/container-registry@069a7051))