Permissioning v2: bind to custom smart contract implementation

[nmvalera]

Summary

Currently smart contract used for permissioning v2 are tightly coupled to GoQuorum code which does not allow users to possibly use a custom implementation of permissioning smart contracts.

Acceptance criteria

• [ ] Allow GoQuorum to read permissions from a custom smart contract implementation matching permissioning v2 interface
• [ ] Standardize Events so cache can be updated from a custom implementation contract (a possible option would be to allow GoQuorum to bind to arbitrary contracts emitting events from a standardized set of permissions event)
• [ ] Make cache optional
• [ ] Enable migration from one smart contract implementation to the other
• [ ] Permissioning configuration should be updated to accept minimally the permissioning contract address, optionally accept other contracts emitting events for cache

[LeoHLee]

Looking forwarding to this! Is this work still in progress?