This new approach will include two different ways to create OPS plugins. The key difference is in how plugins are discovered:

1. "File plugins": This is the same functionality that has already been implemented: search specific directories for files that can be used as plugins.
2. "Namespace package plugins": This approach is basically described in https://packaging.python.org/guides/creating-and-discovering-plugins/#using-namespace-packages. We'll expect native namespace packages.

"File plugins" are suitable as a quick-and-dirty approach for yourself or for small-scale sharing. "Namespace package plugins" take a little more work, but are more suitable to be widely distributed.

This PR will include support and testing for both plugin types.