Let the max_gas() function in BuiltinActor's take complexity argument(s)

[ekovalev]

In the modification made to the BuiltinActor trait (#4345), the new max_gas() function - a counterpart of the "weight()" function for pallets extrinsics, does not accept any arguments, so it can only report a single maximum gas needed for any message processing, regardless of the concrete payload.
To fine-tune this behaviour, we might want to let it take some complexity arguments the way extrinsics weight functions do, so that we can have more accurate estimation of the expected gas budged based on the actual message payload.

This, however, may require further refactoring of the BuiltinActor trait.