Add a save function to the session sidecar

[olevski]

This can be used by the ui to save unsaved work in a user's session when a user tries to shut down a session that has unsaved work.

[olevski]

This would run git add, commit and push.

Alternatively we can also install the renku-cli in the git sidecar and use renku save for this purpose.

[andre-code]

Please also include the Refresh function.
Ref: SwissDataScienceCenter/renku-ui#1956

[olevski]

@andre-code #1196 is for "refresh"/git pull

[ciyer]

The implementation will require a bit more than just renku save. It will need to do the following:

- git fetch
- git status --porcelain --branch: Should return that checkout is at or ahead of upstream, if not fail
- renku save

The second command should provide user-understandable feedback as to why the save couldn't run if it fails at that point.