Treat AmberTools as an optional dependency

[mattwthompson]

Is your feature request related to a problem? Please describe.

AmberTools is no longer used to assign AM1-BCC partial charges in mainline OpenFF force fields. This is expected to be the case in the indefinite future.

We should treat AmberTools as an optional dependency and update packaging to reflect this.

Describe the solution you'd like

Don't assume in source code that AmberTools is installed, also don't install it by default in packaging.

Describe alternatives you've considered

• Continue to use old versions of other upstream packages (Boost and NetCDF come to mind) because of compiled incompatibilities
• Continue to maintain AmberTools package

Additional context

Related #2178

[j-wags]

We've removed AmberTools as a requirement from openff-toolkit packaging, but the next step on this would be to make a new slot in the test matrix that exercises the case of no AmberTools AND no OpenEye.