Endpoint for returning the ActiveDoc(s) for a specific service

Add an endpoint to 3scale API to return Activedocs for a specific service.

3Scale APIs has a "list all Activedocs" endpoint that returns all ActiveDocs with "body" attribute that has the ActiveDocs for each service.

There is a way to get from the 3scale "admin/api/services/

{service_id}" endpoint, but no way to get the ActiveDoc for a specific service. For example, some thing like:

GET admin/api/services/{service_id}

/activedocs/ -List all the activedocs associated with a service
GET admin/api/services/

{ service_id}

/activedocs/

{active_doc_id}

- Get a specific activedoc

In the developer portal's Liquid reference documentation there is a service drop, which allows one to list active docs for a service with the "api_specs" attribute. The thinking is that there might be an endpoint in 3scale API that provides this capability too.