Implement RunningKernel trait for native and remote kernels (#20934)

This PR introduces a unified interface for both native and remote
kernels through the `RunningKernel` trait. When either the native kernel
or the remote kernels are started, they return a `Box<dyn
RunningKernel>` to make it easier to work with the session. As a bonus
of this refactor, I've dropped some of the mpsc channels to instead opt
for passing messages directly to `session.route(message)`. There was a
lot of simplification of `Session` by moving responsibilities to
`NativeRunningKernel`.

No release notes yet until this is finalized.

* [x] Detect remote kernelspecs from configured remote servers
* [x] Launch kernel on demand

For now, this allows you to set env vars `JUPYTER_SERVER` and
`JUPYTER_TOKEN` to access a remote server. `JUPYTER_SERVER` should be a
base path like `http://localhost:8888` or
`https://notebooks.gesis.org/binder/jupyter/user/rubydata-binder-w6igpy4l/`

Release Notes:

- N/A