BE implementation - MR Creation Flow for SAST UI Config Changes

This is the issue to track the backend implementation of #220579 (closed).

Implementation plan and all requirements conversations should be captured in that issue.

Implementation Plan

REST API

• A new endpoint to POST the new SAST Config to will be created
• This will create a new branch (based off the default branch) and new commit
• A url pointing to the New Merge Request page will be returned to allow a new MR to be created

Further considerations:

• This will only happen when there is no existing .gitlab-ci.yml file
• The first iteration does not need to support processing CI variables supplied in the POST call
• We will need to change behavior based on whether AutoDevOps is enabled or not (the .gitlab-ci.yml file will reference the AutoDevOps vendored template or the SAST vendored template)