Add support for passing extra arguments to urllib3 and requests (including timeouts and certificate verification)

This stack of changes adds support for setting timeouts and disabling certificate verification.

To do so, we introduce generic urllib3 and requests kwargs handling (for both the smart and dumb loader), then we add a specialized shortcut for timeouts and certificate verification, which are probably going to be the most used parameters.

This sets default timeouts to 120s for connect and 60s for read, which is better than the current lack of timeout.