FFI should allow for arbitrary connectors

[kitlith]

Feature Description

Right now, we're restricted to the connectors included within the rust library.

On the rust side, we should add a FFI connector struct that wraps a few callbacks passed over FFI. This should be exposed somehow, maybe as its own dedicated function? (buttplug_create_connector?)

Open Question: how are the messages communicated across this FFI boundary? Can they be expressed in terms of the protobufs we already use?