Register clients with user_type "bot"

Depends on https://github.com/matrix-org/synapse/pull/5902

This means that synapse servers that require consent will not demand it from bot users.

maubot/management/api/client_auth.py

@@ -93,6 +93,7 @@ async def register(request: web.Request) -> web.Response:
             "password": password,
             "admin": False,
             "mac": mac,
+            "user_type": "bot",
         }))
     except MatrixRequestError as e:
         return web.json_response({