Move DAST scan service to use existing CI service

What does this MR do?

• swaps out existing mvc on-demand dast scan code to use existing ci services
• pulls dast config from filesystem
• rationale

Related Issues

• #218685 (closed)

How to test this?

• enable the security_on_demand_scans feature flag:

echo "Feature.enable(:security_on_demand_scans_feature_flag)" | rails c

• login
• navigate to graphiql e.g. http://gitlab.localdev:3000/-/graphql-explorer
• run the following mutation replacing the placeholder values

mutation {
  runDastScan(input: {projectPath: "placeholder", branch: "master", targetUrl: "placeholder", scanType: PASSIVE}) {
    pipelineUrl
  }
}

• navigate to the pipelineUrl that gets returned

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry behind feature flag
• 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