Add min_signers to PublicKeyPackage

[conradoplg]

James Katz pointed out in Discord that PublicKeyPackage does not have a min_signers. This prevents a non-Participant Coordinator from succesfully running the protocol since they will have no idea how many participants need to be involved in a signing session.

Add a min_signers field to the PublicKeyPackage.

This is a breaking change so it will need to be included only in 3.0.0

We must be careful to not break serialization compatibility if possible (we should not break that even in major versions if possible). That might be achievable using an Option. If it's not possible, that will require bumping the serialization version which will be a pain.

[StackOverflowExcept1on]

It would also be nice to add getters fn max_signers(&self) -> u16 and fn min_signers(&self) -> u16 for PublicKeyPackage. max_signers seems like it might already be known from self.verifying_shares.len(). I also ran into this in implementation of the ROAST protocol: https://github.com/StackOverflowExcept1on/roast/blob/70dcfee21fe45979b361df107ea81cf7290c0cc7/roast-core/src/coordinator.rs#L60-L66