Add ProxyRequestTimeout option for long-running backend requests

Vulcand was not setting any timeouts in the httploc package inside the vulcan library and so requests which might take more than 10s to serve from the backends were timing out.

This PR adds an option to set the timeout in proxy requests, and sets vulcand's writeTimeout to that same timeout if it exists.
Instead of introducing a new constructor, I would just add these new timeouts to a struct Options{} and pass it to the `NewConfigurator`. These parameters will be always set via command line via defaults or explicitly