ERC-4337 & ERC-7579 compliant modular smart accounts with ERC7579 signer/validator support

[Amxx]

Variant of #4991 with full 7579 based userop verification

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[ernestognw]

I've been reviewing and it's looking good if we only consider the ERC7579 view of how to implement accounts.

I'd like to challenge the idea that ERC7579 is indeed the minimal version of an account; IMO, the validators are not a good fit for identities since they're not something you change frequently as you may do with a module. That would require a lot of operations during deployment (e.g. deploying multiple identities and enabling multiple modules).

From my point of view, identities are core to the functionality and we already have a standard for signature interoperability (ERC1271) so I'd like the identities approach to be more focused on making ERC1271 work out of the box rather than through a validator module.

I opened this PR to demonstrate how it simpler if we allow a generic identity to be plugged into an account (see AccountIdentity) or multiple (AccountMultiIdentity). I feel that approach is way leaner, but we can still write ERC7579 modules for more complex requirements (e.g. Governor as an executor).