Support for openff-bespokefit

[jmichel80]

It would be desirable to make it easier for users to combine molecules parameterised with openff-bespokefit with other system components. This would lower barriers for benchmarking studies with forcefield parameters optimised for each ligand which should be considered the current SOTA for classical force field FEP studies.

This could be done by wrapping around openff code described here and extending the BioSimSpace.Parameters API

At present interchange may be used to export a parameterised molecule into a file format that is compatible with a Sire parser. Longer term a direct openmm->sire conversion may be more robust.