[4337] User Operation with Custom Singleton Signers

[nlordell]

Fixes #148

This PR introduces an E2E test using the official bundler where we deploy a Safe that uses a custom (and silly 🤪) signing scheme over ERC-1271. Specifically, the signing scheme is deployed as a singleton contract that can be shared by multiple Safes using a mapping(safe => data) to differentiate between signers. In theory, this could be adapted to, for example, checking a signature using P256 curve in order to integrate with WebAuthn, and is meant to “prove” that the Safe with the ERC-4337 module would work with existing WebAuthn signer implementations.

The actual implementation is a little convoluted for a few reasons. Namely, ERC-4337 only allows a single CREATE2 per user operation that MUST result in the userOp.sender account. Because of this, deploying ad-hoc signers in the Safe.setup call is not possible, and we have instead opted for a “singleton signer” pattern, where the logic for verifying signatures is shared and a mapping is used to store per-Safe context relevant to signature verification.

Additionally, In order for this to work with ERC-4337, a staked factory is needed (as accessing associated storage for user operations with initCode require a factory). Unfortunately, staked factories add all kinds of complexities:

• Currently, the staked factory contract is quite dumb (can only stake, but can’t unstake). A full staked factory contract requires us to iron out some requirements and make a proper implementation with documentation and tests (out of scope for this PR and issue, and a non-trivial amount of work)
• Staking is determined per bundler, so while some amount of stake may be sufficient for a bundler, it may not be sufficient for all (which adds additional off-chain management dependencies).

For these reasons, I would recommend exploring #149.

[coveralls]

Pull Request Test Coverage Report for Build 6967672785

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 6967652298:	0.0%
Covered Lines:	29
Relevant Lines:	29

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💛 - Coveralls

[nlordell]

This is just to verify this model would work for P256 signatures (which require 2 uint256 slots, one for the X and one for the Y coordinate of the public key - which is a point on the curve).