Changes to the GBSA spec and implementation

[j-wags]

Is your feature request related to a problem? Please describe.
Per "Open questions" in openforcefield/openff-toolkit#363 (comment), there are several more things we could do to make a more flexible GBSAHandler class. However, these will involve creating a CustomGBForce exposing the values we choose.

This issue will serve as a location to discuss these open questions, changes to the GBSA spec, strategies for implementation, and how/whether to make the Force transferable to other simulation packages.

[mattwthompson]

Since any implementation changes appear to be blocked by specification changes, I'm triaging this to the standards repository.