Specify partial charge generation method for small molecules

[ijpulidos]

The openff toolkit allows specifying different methods for assigning partial charges to the molecules via keyword arguments. Should openmmforcefields add support for specifying the backend/wrapper and the method when parameterizing molecules?

This could also help downstream tools, such as in https://github.com/OpenFreeEnergy/openfe/pull/598

We currently have some machinery in place for the EspalomaTemplateGenerator, we would need to extend it for the other template generators. Is it the SMIRNOFFTemplateGenerator in this case?