Add way to store data returned from task

For example when a job created from a task such as:

@app.task(pass_context=True)
async def hello_world(ctx):
    return "Hello World"

finishes successfully, then the “Hello World” should be able to be looked up using the job id.

[ewjoachim]

We might do something more straightforward, but until then: https://procrastinate.readthedocs.io/en/stable/howto/middleware.html

[canadaduane]

Thanks!