(Possibly) provide a unified interface for transitions

[penelopeysm]

As discussed in https://github.com/TuringLang/Turing.jl/pull/2304, it may be worthwhile implementing an AbstractTransition supertype in this library and defining a basic interface that exposes the key information in a transition.

Other libraries in the Turing ecosystem (e.g. Turing itself, AdvancedMH, AdvancedHMC) can then either directly subtype this or make a specialised implementation of Transition if necessary. Right now, all of those implement their own Transition types.