Upgradability of Mina contracts

The Bridge, Manager, and ValidatorManager Mina contracts use default permissions, which include Permission.VerificationKey.signature() for setVerificationKey. This makes it possible for the deployer of these contracts to change the contract's verification key and thus change the behavior of the contract.

This introduces centralization risks that users should be aware of, as it grants a single point of control over the system. We recommend that this be clearly documented for users so that they are aware of the extent of the owner's control over the contract. This can help users make informed decisions about their participation in the project. Additionally, clear communication about the circumstances in which the owner may exercise these powers can help build trust and transparency with users.