Add correct context to signing

[marsella]

In the original draft of InnerProtocolParticipant for signing, we only added generic shared context (like, participant set, sid, etc). However, in the paper, presigning gets the full ssid, which includes lots of additional context (the public key shares, the public aux info, the rid).

In #425, we'll likely add the public key shares in order to be able to verify the signature. However, the other elements are still missing.

This issue is not high priority because, in the absence of identifiable abort, we don't actually construct ZK proofs in signing so it's not an active problem to not include them at this point. However, we should add them for completeness.