Create a helper to create/update/delete new destination models in mutations

Create new ExternalStreamingDestination models when any old ones are created.

When these models are created:

• InstanceExternalAuditEventDestination - ee/app/graphql/mutations/audit_events/instance_external_audit_event_destinations/create.rb
• ExternalAuditEventDestination - ee/app/graphql/mutations/audit_events/external_audit_event_destinations/create.rb

When creating destinations:

• Use the newly created helper to accomplish create, delete, update
• New models should have a link back to the original model (id, category composite key)
• It should be behind the feature flag: audit_events_external_destination_streamer_consolidation_refactor

When updating destinations:

• Find the associated model using the _ref column if possible, and update attributes
• If there is no associated model, we do not create the associated model.

When deleting destinations:

• If the deleted destination exists in the associated model's table, delete it.

Reasoning:

• Did not want to create hooks in the db models as a strong dependency when deprecating the old models
• Make it easy to add/remove and use the feature flag
• When updating, we do not create models, as un-created destinations will be done through a migration.

Assumptions:

• Users will only interact with the models via API or UI

Doc: https://docs.google.com/document/d/1gOVM_gMQH-mmBCGaME8Ds1FVZkYgxscXQjq9CkGI5s8