Addition of forcefield version selection for `GAFF` and `SMIRNOFF`

[talagayev]

Describe the feature you would like to have

Currently it is the case that for both gaff and openff the small molecule force fields are hard coded in generate_forcefield and generate_transitional_forcefield it would be good to make a separate function for them, with adding the versions and adding them in OpenMMDL Setup for the user to select, with the default being the latest version.

Additional context