Library: hot-adding/removing port-forwards

[aramperes]

Add and remove port-forwarding routes at runtime when using onetun as a library embedded in another application.

At the moment, I'm envisioning this being an event that can be passed in the Bus, and be picked up by the various endpoints to create the necessary sockets or shutdown active connections.

I will wait for #6 to be completed, as it may affect the feasibility of this feature.

Originally suggested by @jkcoxson in #35