BUM-2705 Update Server-Side SDKs To Support Session ID Response + tests.

Signed and unsigned calls are wrapped in an associative array containing two elements:
   - result_obj: An associative array representing the deserialised JSON response.
   - session_key: valid session key for a user.
Can we change `result_obj` to something like `response` instead? It just makes it clearer two things:

* That depending on the content-type, this may not be an object.
* That this is the response to the HTTP request that was made, and the rest of the fields are supplementary information.