Use legacy destination helper in legacy destination create APIs

What does this MR do and why?

Uses the helper created in this MR !175628 (merged)

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Before	After

How to set up and validate locally

To test:

• Checkout this branch locally, update migrations, and open gdk -- Ensure you have an Ultimate/EE license

• Enable the feature flag audit_events_external_destination_streamer_consolidation_refactor

• For legacy -> stream destination: you can use the UI following the documentation here to do so:

  • https://docs.gitlab.com/ee/user/compliance/audit_event_streaming.html
  • https://docs.gitlab.com/ee/administration/audit_event_streaming/index.html
  • When you create it through the UI, open gdk rails console and look for the new models: AuditEvents::Group::ExternalStreamingDestination or AuditEvents::Instance::ExternalStreamingDestination depending on which you've created

• To test the reverse flow, open http://localhost:3000/-/graphql-explorer on your gdk instance.

EX:

mutation CreateInstanceAuditEventStreamingDestination {
  instanceAuditEventStreamingDestinationsCreate(
    input: {
      config: {
        googleProjectIdName: "google-project-id-name",
        clientEmail: "clientEmail@example.com",
        logIdName: "gcp-log-id-name"
      }, 
      category: "gcp", 
      name: "Test Local Destination API GCP 12345"
      secretToken:"SecretToken12345!" # required
    }
  ) {
    externalAuditEventDestination {
      id
      name
      category
      config
      secretToken
    }
    errors
  }
}

mutation CreateGroupAuditEventStreamingDestination {
  groupAuditEventStreamingDestinationsCreate(
    input: {
      groupPath:"twitter",
      config: {
        googleProjectIdName: "google-project-id-name",
        clientEmail: "clientEmail@example.com",
        logIdName: "gcp-log-id-name"
      }, 
      category: "gcp", 
      name: "Test Local Destination API GCP 12345"
      secretToken:"SecretToken12345!" # required
    }
  ) {
    externalAuditEventDestination {
      id
      name
      group {
        fullName
        fullPath
      }
      category
      config
      secretToken
    }
    errors
  }
}

Configs are different for each category, and they can be found from their json schemas here:

• HTTP: https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/validators/json_schemas/audit_events_http_external_streaming_destination_config.json
• GCP: https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/validators/json_schemas/audit_events_gcp_external_streaming_destination_config.json
• AWS: https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/validators/json_schemas/audit_events_aws_external_streaming_destination_config.json

Example demo where I create a new streaming model through API -> it shows up as a legacy destination as well: Screen_Recording_2024-12-04_at_2.00.12_PM

Example demo where I create a legacy instance destination for HTTP and it shows up as a stream destination via API: Screen_Recording_2024-12-04_at_2.05.40_PM