Problem Description: 

During "approval" task (regardless if is manual or auto approval)
or as a subtask in the same task group
if a `settings.COLLECTION_SIGNING_SERVICE` is defined.

1. Query pulp.SigningService to load the pre-configured signing service
2. If the collection artifact is stored in an external storage it needs to be
   downloaded locally in a temporary location.
3. Invoke the sign method on the SigningService to produce the signature
   Artifact for the collection content.
4. Store the SignatureContentType in the same repo where the
   collection is included.
5. Approve the collection and move to published repository.

QA Criteria: 

• Ensure an approved collection has a signature when the Signing Service is Enabled
• Ensure that on a system without an enabled signing service, approved collection is not signed

Reminder: Backend task