Add Managed Sources /add and /remove endpoints

**New Feature**
 Added support for four new Managed Sources API endpoints to improve usability of the Managed Sources API, and make it easier to add or remove resources or authentication tokens
 * /source/auth/add
 * /source/auth/remove
 * /source/resource/add
 * /source/resource/remove
 
**Improvements**
 Ensure all POST and PUT API requests are sent JSON encoded with correct headers