SG: Add a proper secret api that can be reused across commands

Created by: jhchabran

This adds a generic API to store tokens across the various commands.

Because each commands are their own domain rfc, (and the upcomingslack, ci) the secret store delegates to those packages how to handle serialisation. The store persists and loads those "bucket" from the disk and provide a convenient Get(key, &yourStruct) to access the data.

I added along the way a small function that migrates the old secrets into the new one seamlessly.