Native VPs should account for cpu cycles spent during validation

[sug0]

Just as a note, I think this pattern should be shared across all native vps given the impossibility of tracking their execution at runtime. If a native vp is computationally expensive (access to storage are already correctly tracked though host functions, so no need to care about them) we should extract the complex part out to a host function to which we can assign a gas cost

Originally posted by @grarco in #1923 (comment)

[grarco]

Actually, if the expensive part of a native vp doesn't need to be shared with a transaction or a non-native vp, there's no need to export it to a host function: we can just export it to a separate function, benchmark its execution and charge the gas before calling it