update protocol participants to take participant configs

[marsella]

Right now, the ProtocolParticipant constructor takes one participant identifier for itself and a separate array for the other participants, and we don't validate anything about those types in the constructors. Instead, we should use the ParticipantConfig type, which bundles the self and others into a named type and does validation on construction that there aren't duplicates, there are a valid number of others, etc.

• Update the ProtocolParticipant trait to take a ParticipantConfig instead of separate id and other_participant_ids fields.
• Update all the call sites