JupyterServer: easier switching between extensions

[oliver-sanders]

One JupyterServer instance may run multiple extensions each with their own root endpoint e.g. /cylc and /lab.

We would like to make it easier to switch between different server extensions:

• To help users discover which extensions are available.
• To make it easier to switch away from the default extension (configured in a ServerApp config).

This is a broader Jupyter problem, not just a Cylc thing, but this control must be implemented in every extension separately.

If there isn't an easy interface for listing active extensions / endpoints consider contributing one upstream to jupyter_server to help with this endeavour.

See #298