Refactor gitlab clients

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR updates and refactors gitlab clients to be better inline with the APIs they represent.

• Renames the clients to be plural
• Moves the issue client to gitlab_client folder
• All clients now inherit from GitLabClient
• Renames FailedJobs to JobsClient and make it generic.
• Adds find and create_merge_request methods to merge request client and also add related specs (These will be used in an upcoming MR)

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.