add typing for endpoint

[PietroPasotti]

if typing.TYPE_CHECKING:
    from typing import TypedDict  # if the smallest python version we support has it! Otherwise, you could wrap this all in a try/except and if there's an error define `_EndpointDict = dict` and that's it.
    
    class _EndpointDict(TypedDict):  
        path:str
         port:int # ?

-        metrics_endpoints: List[dict] = [DEFAULT_METRICS_ENDPOINT],
+        metrics_endpoints: List["_EndpointDict"] = [DEFAULT_METRICS_ENDPOINT],      

Originally posted by @PietroPasotti in #95 (comment)

[rbarry82]

Fortunately, since it's all clobbered into a venv for the charm, and we don't rely on anything compiled, we may even be able to get away with it!