Reduce complexity and increase privacy by retooling variable locations/values

[csuwildcat]

To ensure contacts/relationship information is not leaked to unprivileged querying callers, retool the message structure and contextual/ID logic to incorporate the following:

{ // message
  PROCESSING: {
    nonce: "12345675344578547565332537545587978",
    recipients: [Bob],
    descriptorCid: "QmbWqxBEKC3P8tqsKc98xmWNzrzDtRLMiMPL8wBuTGsMnz"
  },
  AUTHZ: {
    author: Alice,
    target: Carol   // If not present, implied storeAt/target can be Alice
  }
}

1. Remove target/recipient from descriptor, to avoid contact leakage.
2. Move target to AUTHZ
   • To enable storage in different destinations without touching the descriptor.
   • Computed message/canonical ID remains the same even if AUTHZ is a different JWS.
3. Consider making context/canonical ID the CID of the PROCESSING object.
4. Nonce in PROCESSING protects against iterability of the author and the recipient(s) by outside querying parties who are given the contextId