[GitOps] Support controls as a separate YAML file

[getvictor]

When originally designed and implemented, gitops was supposed to support controls settings from a separate file. This would make copying team files simpler, as all the copies could reuse the same controls settings.

In team YAML:

controls:
  path: ../lib/common-controls.yml

However, this functionality was not documented and currently has some bugs, such as #24674

Engineering work would be relatively small since this feature is already implemented and has many unit tests.