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/gatekeeper.py

@@ -0,0 +1,32 @@
+import re
+
+SUBSCRIPTION_SLOTS = 100
+
+# TODO: UNITTEST
+class Gatekeeper:
+    def __init__(self):
+        self.registered_users = {}
+
+    @staticmethod
+    def check_user_pk(user_pk):
+        """
+        Checks if a given value is a 33-byte hex encoded string.
+
+        Args:
+            user_pk(:mod:`str`): the value to be checked.
+
+        Returns:
+            :obj:`bool`: Whether or not the value matches the format.
+        """
+        return isinstance(user_pk, str) and re.match(r"^[0-9A-Fa-f]{66}$", user_pk) is not None
+
+    def add_update_user(self, user_pk):
+        if not self.check_user_pk(user_pk):
+            raise ValueError("provided public key does not match expected format (33-byte hex string)")
+
+        if user_pk not in self.registered_users:
+            self.registered_users[user_pk] = SUBSCRIPTION_SLOTS
+        else:
+            self.registered_users[user_pk] += SUBSCRIPTION_SLOTS
+
+        return self.registered_users[user_pk]