20868 - Includes External Files in Gitlab CI YML

What does this MR do?

Enables include keyword on gitlab-ci.yml, allowing the inclusion of external files resulting in a composed file that is evaluated as a single one

The external files can be:

• local to the same repository, referenced using the path
• remote in a different location, accessed using HTTP(S) protocol, referenced using the full URL

The include keyword supports a single local/remote value or an array of multiple local/remote values:

Examples:

include: '/templates/.gitlab-ci-templates.yml'

include: 'https://gitlab.com/gitlab-org/gitlab-ce/blob/8efdf75b/.gitlab-ci-templates.yml'

include:
 - '/templates/.gitlab-ci-templates-1.yml'
 - '/templates/.gitlab-ci-templates-2.yml'
 - 'https://gitlab.com/gitlab-org/gitlab-ce/blob/8efdf75b/.gitlab-ci-templates.yml'

Are there points in the code the reviewer needs to double check?

• Double check if the current implementation covers all the possible scenarios

Why was this MR needed?

To make gitlab-ci.yml structure restful

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #20868