Add support for testing pebble services interaction

There is currently no way in ops-testing to test a charm interaction with a pebble service, like restarting it for example: `self._container.restart(self._service_name)`.

The only way to accomplish this is to mock `ops.model.Container.restart`.

It would be nice to have a better way to do this, maybe as an automatic mock in the output state?

```
...

state_out = self.ctx.run(container.pebble_ready_event, state_in)

assert state_out.containers[0].services["ausf"].restarted
```

Moved from https://github.com/canonical/ops-scenario/issues/34

Start by reading the ops-testing container state handling and the interaction with ops.model.Container.restart. Trace the pebble_ready_event flow through state_in and state_out, then determine how a service restart should be represented, such as the proposed services["ausf"].restarted value. Done means charm tests can observe a restart without mocking Container.restart.

Written by the indexing model from the issue text.