feat: remote anchor transaction manager

[smrz2001]

This PR builds on the previous PR to add a remote CAS transaction manager that communicates with the existing 3Box Labs CAS to anchor Merkle tree roots while hiding from CAS that this is not a normal stream.

The stream ID used for the anchor request is the node public key (from the ED25519 private key) encoded as a CID in an unloadable stream ID.