Add task retrying and backoff

[codingjoe]

Let's backport the current propoal: django/new-features#142

[codingjoe]

I want a minimal API:

def retry_fn(context: TaskContext): -> datetime.timedelta | None
 """Return the delaty until the next execution or None to end retries."""

This can be accompanied by handy backoff utilities. And importantly, the TaskContext.task_result needs to include the current error and should already be updated as failed.