Add support for GitLab

Please describe the enhancement

I would look forward if Frizbee could support GitLab .gitlab-ci.yml files.
There two options which would benefit from the support are:

Job Images:
Here we declare a Container Image and its version tag (e.g. latest, v4.2.0).

Includes and CI/CD Components:
Here we declare a CI/CD Component, essentially a pre-defined job which should be run in the project and it's version

Solution Proposal

Frizbee should be able to look into .gitlab-ci.yml files and provide the checksum for declared Job images and included components.

Describe alternatives you've considered

Not supporting it?!

Additional context

No response

Acceptance Criteria

I am not sure about how much more difficult the support for includes are, therefore I would consider this feature done if GitLab Job images can be supported.

Start by locating the existing configuration parsing and checksum-resolution entry points, then inspect how .gitlab-ci.yml job images are represented. The minimum done state is support for declared GitLab job images and their checksum output; includes and CI/CD components are optional according to the acceptance criteria.