working copy - allow to custom save method on model (#172345)

[bpasero]

Part of #172345

This allows a stored file working copy model to implement the save method. Notebooks could opt-in to use this method, if:

• the notebook is stored remotely (to improve perf)
• maybe behind a setting in the beginning or only enabled in insiders

The model that implements save needs to:

• respect cancellation token
• respect the passed in write options (specifically should support etag validation)
• most importantly make sure that the data that gets saved is in-sync with the notebook editor on the renderer