Allow to include another project files

What does this MR do?

This implements a syntax to include files from another project:

include:
  project: gitlab-org/gitlab-ce
  file: /file # include file with ref that is `HEAD`

or

include:
  - project: gitlab-org/gitlab-ce
    file: /file # include file with ref that is `HEAD`

  - project: gitlab-org/gitlab-ce
    file: /file
    ref: feature-branch

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53903

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer