Add timeout to request config for external routing group selector

[Chaho12]

We are using Airlift HttpClient's default setting to make requests, but in some cases, 5 min seems to be too long when making HTTP requests.
It would better for users to have options to change timeouts.

    private Duration requestTimeout = new Duration(5, MINUTES);
    private Duration idleTimeout = new Duration(1, MINUTES);

https://github.com/airlift/airlift/blob/master/http-client/src/main/java/io/airlift/http/client/HttpClientConfig.java#L69-L70