Ability to build without Python support

[elliotcmorris]

Currently, the resolver assumes a python environment, ergo, it needs to be run from a python application, even though the USD libs that load it are C++.

This isn't neccesarily great, we should consider how to let the resolver run in non-python contexts.

[feltech]

I think this needs to be a build option. E.g. USDOPENASSETIORESOLVER_ENABLE_PYTHON.

We can default the build option value based on whether the discovered USD package dependency supports Python or not (i.e. was build with PXR_ENABLE_PYTHON_SUPPORT).

We should allow manual override, since it is possible that the user wishes to use in an application with Python available, and that uses USD without Python support.

[feltech]

This issue was originally written before the usdOpenAssetIOResolver supported C++ OpenAssetIO plugins, and so was framed as a need to ensure Python was available. I've updated the title/description to instead frame as a need to make Python optional.