Write variables to a new .gitlab-ci.yml

Building on the work in BE implementation - MR Creation Flow for SAST UI Config Changes, we will add support for writing variables to the .gitlab-ci.yml file.

Initial fields to support:

• Image prefix
• Image tag
• excluded paths
• stage
• search maximum depth

Implementation Plan

• Move existing logic for writing the .gitlab-ci.yml out of the controller (this will also be relevant for the move to a GraphQL transformation)
• Add support for writing the above fields