add an opener class for access via fs.open_fs

[d-w-moore]

So that we may instantiate a session with:
fs.open_fs("irods://...")
https://docs.pyfilesystem.org/en/latest/extension.html?highlight=opener#opener

[d-w-moore]

The thing that makes this harder is that we don't have a setup.py, which is the normal place for making entry points. Perhaps the pyproject.toml way of doing things allows something similar?

[hechth]

@d-w-moore yes, it does - we can provide an entry point :)