Key ceremony data implementation

In the feat/braid-0.4.0/main branch, the key ceremony data downloaded by the trustee contains:

• The Channel artifact, which includes the encrypted private key for peer-to-peer trustee share communication
• All the Shares artifacts, each containing the corresponding share sent to the downloading trustee.

The Share artifacts contain shares sent to the trustee, encrypted with the trustee's public key, (also in the Channel). These shares can be decrypted with the private key in the Channel. This private key is encrypted with a symmetric cipher, whose symmetric key is stored in the trustee's secure storage area.