Add request cookies to login flow context dict

Adds cookies from the aiohttp Request object associated with the request to POST
/auth/login_flow.  This is useful if an auth provider operates on data provided
through cookies by an external system.

Example use-case:

  * Reverse proxy handles authentication ands sets a JWT in a cookie upon
    success.  HomeAssistant can allow access after validating this JWT.

homeassistant/components/auth/login_flow.py

@@ -177,6 +177,7 @@ async def post(self, request, data):
                 handler,
                 context={
                     "ip_address": request[KEY_REAL_IP],
+                    "cookies": request.cookies,
                     "credential_only": data.get("type") == "link_user",
                 },
             )