Adds basic register logic

- 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

teos/errors.py

@@ -1,4 +1,4 @@
-# Appointment errors
+# Appointment errors [-1, -64]
 APPOINTMENT_EMPTY_FIELD = -1
 APPOINTMENT_WRONG_FIELD_TYPE = -2
 APPOINTMENT_WRONG_FIELD_SIZE = -3
@@ -8,6 +8,10 @@ APPOINTMENT_FIELD_TOO_BIG = -6
 APPOINTMENT_WRONG_FIELD = -7
 APPOINTMENT_INVALID_SIGNATURE = -8
 
+# Registration errors [-65, -128]
+REGISTRATION_MISSING_FIELD = -65
+REGISTRATION_WRONG_FIELD_FORMAT = -66
+
 # Custom RPC errors
 RPC_TX_REORGED_AFTER_BROADCAST = -98