Add a SNARK verification key Proof of Bound Possession

## Why

We want to make sure that no registered SNARK verification key can be used by an adversary to pollute the membership registration. A way to avoid this problem is to implement a Proof of Bound Possession for the SNARK verification key.

## What

Implement a Proof of Bound Possession for the SNARK verification keys.

## How

- [x] Create a document explaining the Proof of Bound Possession mechanism
- [x] Add the SNARK Proof of Bound Possession to the SNARK verification keys
- [x] Adapt the SNARK key registration to not return errors on collision of verification key (with a deterministic resolution in case of collision)
- [x] Make backward compatibility for codecs of the signer registration messages
- [ ] Check that computation of total stake is accurate for the SNARK AVK