WIP: Reuse DAST CI template for on-demand DAST scan

What does this MR do?

this merge request swaps the hard coded dast scan config out and replaces it by pulling the requisite data from the ci template. the reason behind this is that we don't want to have to maintain this in two places.

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 toggle
• 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