[FEATURE] Return HTTP Status from Extensions

[dbwiddis]

Is your feature request related to a problem?

As part of #64, OS #4282 established a basic way for extensions to respond to REST requests with a String.

To expand functionality, the return value from extensions should include the HTTP status (RestStatus enum) along with a String.

What solution would you like?

The ideal return type would be BytesRestResponse but this is not configured to be passed over Transport, so a new ExtensionRestResponse object should be created.

What alternatives have you considered?

Adding the enum value at the beginning of the string with a delimiter. This would work but it's a hack and we should do it right!

Do you have any additional context?

See first comment at #109.