Added a `start/end` system where tracer can be notified that processing of some Ethereum system calls is starting processing and also notifies it when the processing has completed.

Doing a `start/end` for system call will enable tracers to "route" incoming next tracing events to go to a separate bucket than other EVM calls. Those not interested by this fact can simply avoid registering the hooks.

The EVM call is going to be traced normally afterward between the signals provided by those 2 new hooks but outside of a transaction context `OnTxStart/End`. That something implementors of live tracers will need to be aware of (since only "trx tracers" are not concerned by `ProcessBeaconRoot`).

One thing of importance for tracers