Removing `nbconvert` as a required dependency

[jamesbraza]

Problem

I have jupyterlab as a dependency, which depends on jupyter-server, which installs nbconvert.

Can we move nbconvert to an optional dependency via an extra?

The source code in https://github.com/jupyter-server/jupyter_server/tree/v2.12.5/jupyter_server/nbconvert already doesn't require it at import time, so the code is already compatible with this.

It seems https://github.com/jupyter-server/jupyter_server/issues/15 is related.

Proposed Solution

1. Moving nbconvert to an optional extra
2. Adding nbconvert as a test dependency

Additional context

N/a