Following the same apporach as the Inspector, the gatekeeper now raises exceptions depending on the error encountered.
Also generalises `identify_user` so it can be used with any kind of user-signed message.
Finally lifts the restriction of having available slots on `identify_user` (that only applied to `add_appointent`).
+ Users need to be registered in order to send appointments (free registration for now)
+ The tower gives them a number of appointments to work with
+ Non-registered users and users with no enough appoitnemnts slots return the same error (to avoid proving)
- Authentication does not cover get_* requests yet
- No tests
- No docs
- Adds register endpoint in the API
- Adds the Gatekeeper to keep track of registered user and allow/reject access
- Adds registration errors
- Updates API unit tests
- Refactors some methods of the API to reduce code replication
