Code deduplication in the threshold signatures repo

[SimonRastikian]

Background

Duplication exists for the first four checks in presign (presign.rs:58-80) and in assert_sign_inputs of sign.rs:

1. participants.len() < 2 → NotEnoughParticipants
2. ParticipantList::new(...) → DuplicateParticipants
3. contains(me) → MissingParticipant { role: "self" }
4. threshold.value() > participants.len() → ThresholdTooLarge

Goal: reduce the code redundancy.

User Story

No response

Acceptance Criteria

Make a shared function assert_participant_inputs that would be called as a helper in presigning and signing

Resources & Additional Notes

That was spotted by Barak #3343 (comment)