Reduce the number of API calls to K8S

The charm makes a large number of API calls to retrieve information
about the IP address allocated to the K8S service; cache this
information to reduce the number of calls made and speed up hook
performance.

Note functools.lru_cache is used as this charm runs on 20.04 which
uses Python 3.8 - when the base is updated to 22.04 the more simple
cache decorator can be used instead.

src/charm.py

@@ -5,6 +5,7 @@
 """Charm Traefik."""
 import contextlib
 import enum
+import functools
 import ipaddress
 import json
 import logging
@@ -1271,6 +1272,7 @@ def _scrape_jobs(self) -> list:
         ]
 
 
+@functools.lru_cache
 def _get_loadbalancer_status(namespace: str, service_name: str):
     client = Client()  # type: ignore
     traefik_service = client.get(Service, name=service_name, namespace=namespace)