DAST On-demand scans MVC - Initiate Scan - Run scans - Backend

Summary

This issue is for the backend work of #218465 (closed). The backend should be able to run new DAST scans on a given target URL passed by the frontend via a GraphQL mutation.

The proposed GraphQL mutation looks like this:

mutation {
  runDastScan(input: {projectPath: ID!, targetUrl: String!, branch: String!, scanType: DastScanTypeEnum!}) {
    pipelineUrl
  }
}
# where DastScanTypeEnum has a single value: PASSIVE

Implementation plan

• Implement the GraphQL mutation resolver
  • The mutation triggers a new DAST scan attached to the given branch
  • It responds to the frontend with the pipeline's URL (pipelineUrl)
  • Add validation GraphQL error response
  • Add new policy
  • Mark as on-demand scan (avoid migration)
  • Add ability to use YAML string create CI pipelines
  • Move DAST scan service to use existing CI service
  • Get YAML template from FS rather than duplicate