Document and test proxy's soft dependencies

[GeorgianaElena]

The etcd3 and python-consul2 libraries are not installed anymore by default, when installing jupyterhub-traefik-proxy(https://github.com/jupyterhub/traefik-proxy/pull/127). This libs are only required by the key-value store variants of the proxy, so TomlProxy shouldn't need them (checkout https://github.com/jupyterhub/traefik-proxy/issues/125 for more context).

We should:

• [ ] Document this
• [ ] Add a test that checks that TomlProxy doesn't require these libraries and works without them (I suspect some refactoring of the testing infrastructure needs to happen before too, because the proxies are a bit "tied together" in the CI)