Support `ref` for GitOps agnet configs

This change sets adds a new configuration option for GitOps manifests:

gitops:
  manifest_projects:
  - id: root/agentk-test
    default_namespace: test-ns
+    ref: 
+      tag: v1
    paths:
      # Read all YAML files from this directory.
    - glob: '/manifests/*.yaml'

ref takes one of tag, branch or commit.

Closes:

• gitlab-org/gitlab#353101 (closed)
• gitlab-org/gitlab#259671 (closed)
• gitlab-org/gitlab#259670 (closed)
• gitlab-org&4516 (closed)