Add mechanism to trigger state changes to actors

[stephen-hlx]

A robust system should remain correct when it reacts to external events that alters its state.

For instance, to verify that a data pipeline remains correct when there is an influx of data coming from its various input sources:

 ┌─────────┐
 │data_src1├─┐
 └─────────┘ │  ┌─────────────┐
             ├──►data_pipeline│
 ┌─────────┐ │  └─────────────┘
 │data_src2├─┘
 └─────────┘

We need to simulate those input events.
To do that, we may need some actors that only pumps data into the data pipeline.
These data will probably just be regular actors but their state is not checked when verifying the global state.

This seems to belong to a part of the "How to" section in the README?