allow running pre and post-scripts on message receive events and end events

[burnettk]

since receive events act like message receive tasks, they should probably also allow pre and post scripts.
having the same on end events would also be handy, in order to craft final instructions.
are there other events that would want them?

note that there was a related bug on message receive tasks. that has been fixed in the context of #1842 and sartography/SpiffWorkflow@7e15a0b. this is about events rather than tasks.

raised by volker.