Feature Request: Ability to control request timeout > 120 seconds

Currently, if any request takes longer than 120 seconds, the following exception gets raised:
> TimeoutError: Request timed out after 120 seconds.

This appears to be controlled by the `invocation.INVOCATION_TIMEOUT` constant and does not appear tunable. I see that the constructor for `Invocation` takes in a `timeout` parameter that defaults to that constant value, but as far as I can tell there's no way to get a timeout actually passed-in to the `Invocation` object since that's all handled under-the-hood when the client is configured.

I would love it if I could control the timeout on a per-request basis.