[Improvement] De-enshrine `validateUserOp`

[adam-alchemy]

(Previous discussion: community call 1, eth-magicians)

• The special status of validateUserOp in ERC-6900 is key to allowing custom account security settings and automations. However, with proposed changes to ERC-4337’s UserOperation struct and RIP-7560’s validateTransaction, we do not want to be stuck re-enshrining more special functions. We need to define a strategy for functions to have implementation-switching as needed to have modular validation functions.