`/ping` route support HEAD request method

Allow services like Uptimerobot to monitor the WebUI which uses `HEAD` request method because it is lighter whereas in order to use `GET` method, payment is required.

There's no significant impact for this changes.

freqtrade/rpc/api_server/api_v1.py

@@ -71,7 +71,7 @@ router_public = APIRouter()
 router = APIRouter()
 
 
-@router_public.get("/ping", response_model=Ping, tags=["Info"])
+@router_public.api_route("/ping", methods=["GET", "HEAD"], response_model=Ping, tags=["Info"])
 def ping():
     """simple ping"""
     return {"status": "pong"}