OpenGSN Integration

[open-contracts]

Main goal
To facilitate onboarding of new users, allow calls to an Open Contract function (which requiresOracle) from an empty wallet, by integrating with OpenGSN.

Requirements

• Expose a simplified OpenGSN-API-wrapper via the OpenContract parent class
• Integrate the OpenGSN provider with the frontend
• If possible to do securely, one OpenContractsPaymaster should be responsible for all Open Contracts (on a given chain) to mimize the complexity+cost of an individual Open Contract as much as possible.

ToDo
Chronologially, we need to:

• Clarify Specs for OpenContractsPaymaster and OpenContract
• Implement Solidity contracts
• Adapt open-contracts/client-protocol to use OpenGSN

[open-contracts]

Let's add the spec into https://github.com/open-contracts/ethereum-protocol/blob/OpenGSN/OpenGSNSpec.md