Add boresch-specific custom bonded force #765
Description
From OpenFreeEnergy/openfe#1043 it seems pertinent that we should probably add Boresch type of forces/restraints to openmmtools. Specifically what's in https://github.com/OpenFreeEnergy/openfe/pull/1043/files#diff-68c88a73730daf2c6de3f437f3ecad927ff3b31f9235bddd6db4877e2521def4
@IAlibay Please add more details as needed here. Thanks!