Integrate hybrid topology factories into openmmtools

[ijpulidos]

Current implementation of hybrid topology factories in perses are currently in a state that they could be integrated into openmmtools code base. Maybe an alchemy package/module that contains it (for example what has been proposed in the relative branch ).

Namely, this would minimally require migrating the HybridTopologyFactory and the RESTCapableHybridTopologyFactory into openmmtools.

[IAlibay]

Following up from discussions with @jchodera on slack - how would folks feel about a first pass of just moving the base HTF class without the TopologyProposal requirements into openmmtools?

We could take https://github.com/OpenFreeEnergy/openfe/blob/main/openfe/protocols/openmm_rfe/_rfe_utils/relative.py, make sure we merge in any new changes from Perses (I think cutoff modification hasn't been ported through yet). And then start iterating on refactoring the object model?