Support writing variables to CI config

What does this MR do?

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

See issue Write variables to a new .gitlab-ci.yml for more context.

Note: Once BE implementation - Implement GraphQL mutation for MR Creation flow and FE implementation - Convert Rest API to GraphQL mutation for MR Creation flow are implemented, we will remove the #create action of ee/app/controllers/projects/security/sast_configuration_controller.rb

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