Include templates

What does this MR do?

This MR allows to include bundled templates.

This introduces a syntax:

include:
  template: Auto-DevOps.gitlab-ci.yml

OR

include:
  - template: Auto-DevOps.gitlab-ci.yml

It also adds additional syntax for all existing includes:

include:
  remote: http://my-remote/file

OR

include:
  - remote: http://my-remote/file

include:
  local: /local/file.yml

OR

include:
  - local: /local/file.yml

What are the relevant issue numbers?

Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/53445.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• 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