Provide an API to force access to the host

[bsideup]

Before this change, one of the biggest limitations of host port exposing
was that the ports had to be exposed before the container starts.

This change allows forcing starting the forwarding container,
so that host listeners can be started/exposed after starting the consumer.

[kiview]

Super helpful improvement for the API 👍 . We need to update the docs accordingly.

[bsideup]

@kiview added a mention to the docs 👍

[rnorth]

I think I'd suggest making the docs edit a bit more prominent, but this can be changed later and should not stand in the way of releasing this PR.