Initial implementation

Move auth logic out of Card API to a shared library, this is largely a lift-and-shift with some documentation and additional tests, with the code being moved out of: https://gitlab.developers.cam.ac.uk/uis/devops/iam/card-database/card-api/-/merge_requests/90.

Part of https://gitlab.developers.cam.ac.uk/uis/devops/iam/identity-photos-service/photo-api/-/issues/16.

apigatewayauth/api_gateway_auth.py

@@ -24,8 +24,9 @@ class APIGatewayAuthentication(authentication.BaseAuthentication):
     """
     An Authentication provider which interprets the headers provided by the API Gateway.
 
-    The Card API expects to only be deployed behind the API Gateway, and therefore relies on
-    the fact that the headers provided are authoritative.
+    This library expects to only be used within an application that is deployed behind and can
+    only be invoked by the API Gateway, and therefore relies on the fact that the headers
+    provided are authoritative.
 
     """