Build a Round2Package from JSON data.
Note: In the test vectors, participant_data.signing_shares[sender_id] contains
the share that participant receives FROM sender_id.
So for the current participant's round2 packages, we look in the CURRENT
participant's data for the share they receive from each sender.
Build a Round2Package from JSON data. Note: In the test vectors, participant_data.signing_shares[sender_id] contains the share that participant receives FROM sender_id. So for the current participant's round2 packages, we look in the CURRENT participant's data for the share they receive from each sender.