MR Creation Flow for SAST UI Config Changes

What does this MR do?

This MR was branched off of the work in !34947 (diffs)

This work is being done in support of #223878 (closed)

Please note this is an MVC and related functionality that uses this will be behind feature flags. Future iterations will allow the frontend to pass in CI variables that we will inject into the VARIABLES section of the .gitlab-ci.yml file that we are creating. Future iterations will also allow the updating of the VARIABLES section of an existing .gitlab-ci.yml file.

Please also note that we will be moving away from this REST approach with BE implementation - Convert Rest API to GraphQL mutation for MR Creation flow

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team