Add back timestamp and app hash validation in execValidate

[Manav-Aggarwal]

We removed the timestamp and app hash validation in the execValidate method in manager for now due to problems with the evm execution layer.

I saw the timestamp issue when running rollkit with evm execution so need to investigate.

For app hash validation, evm execution uses immediate execution while execValidate is called while assuming deferred execution, so rollkit might need to be aware whether it's immediate or deferred execution to call execValidate correctly or modify execValidate itself to make it aware.

[tac0turtle]

can we leave this check the execution layer? we would extend the execution api and then that way we dont have to worry about it in rollkit itself?