Update EIP-6900: Spec update 2

[adam-alchemy]

• Removed the execution mode enum from IStandardExecutor. Now, it is only expected to use call.
  • Non-standard native account functions that use other call modes, such as delegatecall, are not prohibited by the spec.
• Changed plugin init operation to an array of call operations instead of a delegatecall.
• All plugin function signatures have been collected in one spec section for clarity.
• Pre user op validation functions now return a packed time bound, which is intersected with the validator-returned value.
• Specified behavior for natively-defined functions on the account.
• Specified operation order for updatePlugins (pending plugin function updates do not apply until after execution completes - i.e. after all postExec hooks return and the call frame is complete).
• Updated rationale section, fixed a typo, and updated the two call paths diagram.

[eth-bot]

✅ All reviewers have approved.

[eth-bot]

All Reviewers Have Approved; Performing Automatic Merge...