pre/post save hook registration

[davidbrochart]

Problem

The way pre_save_hook and post_save_hook work is by direct attribute assignment in a FileContentsManager instance, e.g. file_contents_manager.pre_save_hook = my_hook (see the documentation). This doesn't play well when multiple hooks have to be registered e.g. by different server extensions.

Proposed Solution

I was thinking about implementing a setter, that would append hooks to a list under the hood, and then call them in sequence. But they are currently implemented as a traitlet. If we register hooks, that also raises the question of how to unregister them.

Additional context

See jupyterlab/jupyterlab#11599 (comment).