Proof verification guardians

[r0wdy1]

This PR adds yet another layer of security by introducing a set of predefined guardians that would re-do all the proof verification and thus lower the required trust assumptions about honest sequencer.

Architecture

In order to avoid any changes to the Pool contract the PR leverages an already used OperatorManager functionality in a new way: the MPCGuard contract instance is entitled as a sole executor for both transact and appendDirectDeposit methods.
Now in order to be propagated to the Pool the transaction is expected to have a set of additional signatures made by a set of predefined guardians
For the sake of simplicity there are certain limitations and guard rails that could be improved later:

1. The signatures are checked in the same order as the addresses in the guardians' list
2. Both transact and appendDirectDeposit are onlyOperator just as a last line of defense