Authorization support for fetching includes

Problem to Solve

When including external files in your pipeline definition using include keyword, these are fetched using HTTP/HTTPS requests. If you want to access files on another project with no public access (e.g., a private project on GitLab.com) you cannot because we still don't support basic auth schemas in URL, nor variables in the include value.

Proposal

Support inclusion from private projects, and potentially even other similar cases.

Make it possible to specify project and include file path with:

include:
  - project: other-project
    file: file/gitlab-ci.yml