Add a Kentik service to GSO

Add and remove devices using Kentik, the router creation and termination workflows have been updated

gso/services/kentik_client.py

 """The Kentik service is used for external interactions with Kentik."""
 
 import logging
-from typing import Any
+from typing import Any, Literal
 
 import requests
 from pydantic import BaseModel
@@ -39,7 +39,9 @@ class KentikClient:
             "Content-Type": "application/json",
         }
 
-    def _send_request(self, method, endpoint: str, data: dict[str, Any] | None = None) -> Response:
+    def _send_request(
+        self, method: Literal["GET", "POST", "PUT", "DELETE"], endpoint: str, data: dict[str, Any] | None = None
+    ) -> Response:
         url = self.config.api_base + endpoint
         logger.debug("Kentik - Sending request", extra={"method": method, "endpoint": url, "form_data": data})
         result = requests.request(method, url, json=data, headers=self.headers)