feat: [v0.8-develop, experimental] signature validator interface

[adam-alchemy]

Motivation

Previous, ERC-1271 support for 6900 accounts was left up to plugin implementation. This could cause potential function definition conflicts, and we can address this by adding it to the validation interface and supporting multiple validation plugins at once (erc6900/resources#4).

This is an experimental implementation to gather feedback on the proposed change.

Solution

Following the miniMSA pattern of defining isValidSignatureWithSender, rather than the standard 1271 function.
Fully implementing signature validation switching depends on multi-validation support, so that isn't implemented here. Instead, the old defined signature validator is used. This change is intended to cover the expansion to the plugin interface.

[adamegyed]

Closing, as this was superseded by #59