Sync metadata in private sync

[orthecreedence]

When running private sync, there is metadata associated with the identity that would be worth syncing:

• Staged transactions
• Trust assignments
• ephemeral keys (for instance, pre-signed keys for use in MLS groups)

This allows the "owners" of an identity to stage transactions and sync them between themselves so they can be signed and published. This was originally part of #31 but I decided to put off the group identity aspects of Stamp for now. The foundations are there and can be expanded when needed. There are some other more important p2p aspects worth doing first.