Cache git credentials via the jupyterlab-git extension

[rdelubi]

As a feature request:

Can jupyterlab-git extension provide a UI to cache git credentials so that user doesn't have to enter the credentials every time he/she needs to interact with remote repo? e.g. wrap the functionality of "git config credential.helper 'cache --timeout=xxx'" in the extension UI

[fcollonval]

Tasks:

1. First add a settings in the extension to determine if the user wants to cache its credentials:
   1. Add a that settings in schema/plugin.json
   2. Maybe add a other one for the timeout
2. Pass that settings with the user authentication to the backend
3. If the settings needs to be cached
   1. Check if there is already a credentials helper
   2. if not and setting is true add the cache helper

cc @navn-r @SinaKhalili

[meeseeksmachine]

This issue has been mentioned on Jupyter Community Forum. There might be relevant details there:

https://discourse.jupyter.org/t/jupyterlab-git-requires-git-authentication-for-operations-like-stage-unstage-and-diff/11255/2