Add default DS scan profile

What does this MR do and why?

Add default DS scan profile based on Design: Enablement-only dependency scanning pro... (#591553) • Michael Fangman • 18.11

Changelog: changed
EE: true

How to set up and validate locally

1. Select a root group g without DS security scan profiles.
2. Use this GraphQL query to get the available (default) profiles:

   query test {
     group(fullPath: "<g.full_path>") {
       availableSecurityScanProfiles(type: DEPENDENCY_SCANNING) {
         id
         scanType
         name
         description
         gitlabRecommended
         triggers
       }
     }
   }

3. Verify there is no DEPENDENCY_SCANNING profiles available.
4. Use this mutation to attach the default dependency_scanning profile to the projects under group g, and verify it fails with The resource that you are attempting to access does not exist:

       mutation {
      securityScanProfileAttach(
        input: {
          securityScanProfileId: "gid://gitlab/Security::ScanProfile/dependency_scanning"
          groupIds: ["gid://gitlab/Group/<g.id>"]
        }
      ) {
        errors
      }
    }

5. Turn :security_scan_profiles_dependency_scanning feature flag on:
   Feature.enable(:security_scan_profiles_dependency_scanning)
6. Using the same query, verify that default DEPENDENCY_SCANNING profile is returned with the correct name, description and triggers.
7. Using the same mutation, verify you are able to attach the default DEPENDENCY_SCANNING profile to projects under g.

MR acceptance checklist

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Related to [Backend] Add default DS profile (#593009) • Gal Katz • 18.11