* Create package for handling 3pid processes and move invite processing there

* Add database table and functions for tracking 3PIDs

* Add structures and functions to interact with an ID server

* Add handlers for 3PIDs management

* Fix 3PIDs retrieval sending null if no 3PID known for a user

* Include medium in database requests and function calls

* Publish an association if it has been validated and requested

* Add TODO markers for tursted ID server check

* Use a structure instead of a map to represent a 3PID