Add hook to run generic pre- and post-task logic

[james-certn]

I don't have the appropriate environment set up locally (there are no instructions?), so I have not run the test suite changes (they might fail abysmally!)

• aiming to provide a way to hook in app-specific setup before each job task is run
  • have added both pre and post task hooks, for fun.
• the example being to reset the structlog context, to ensure no pollution from one job to the next

TODO:

• add tests
• make documentation changes