OCapN: automerge marshal plugin PoC

[kumavis]

Tagged-based "marshal plugin" for the OCapN client to send and receive Automerge document handles as objects.

This is a proof of concept for marshal plugins

Notes:

• This is a fairly trivial proof of concept, as theres no extra stuff necessary (such as adding the remote to the read access list) to send a document to someone. Doing this for Keyhive may be a bit more interesting as we may be able to automatically add the recipient to the read-access or write-access list as part of the marshal plugin.
• Automerge Documents don't indicate any connection information that could be used to connect to a network that has the document, and I don't attempt to fix that here. This implementation assumes its available via the sync.automerge.org sync server.
• in current implementation they arrive as Promise<DocHandle> (result of repo.find) but could likely be DocHandle if using repo.findClassic or something
• Automerge Document snapshots (handle.doc()) should be json-serializeable plain objects but in actuality don't seem to be Compartment-safe, seems like they might be exposing some internal machinery. hardening such a doc seems to break automerge internal syncing + networking. I did not investigate further.
• the name "pbcap" suggests a "peanut butter + chocolate" mutually beneficial relationship of ocapn refs + keyhive c-caps