Notifies that the body execution has started.
This callback has to return synchronously. It is intended for performing log output,
update
FlowNode, or some such decorative actions. For any asynchronous
computation that needs to happen prior to the body execution, the best place to
do that is before calling
StepContext#newBodyInvoker().
StepContext given to this method lets you access objects that correspond
to the beginning of the body, as opposed to the objects that correspond to the invocation
of the step that invoked the body. Otherwise the context is identical in behaviour
to that given to
Step#start(StepContext).
So for example this is a good place to record any logging that's attributed to
the body execution, such as reporting that this is Nth retry of the body, or
that this is the parallel branch named 'xyz'.