Consider adding a `Duration` enum with `INFINITE` and `INSTANTANEOUS`

[llucax]

We could use special values to represent INFINITE and INSTANTANEOUS for the Dispatch.duration field, so the code gets more readable (without the need for comments) and more type-safe.

We could set the type of duration: timedelta | Duration where duration is a enum for example, then we can also use match like this:

match dispatch.duration:
    case Duration.INFINITE: ...
    case Duration.INSTANTANEOUS: ...
    case timedelta(): ... (here we know it is > 0)
    case unexpected: assert_never(unexpected)

Taking it even a bit further, we could have a PositiveTimedelta object that even guarantee the time is positive.

Originally posted by @llucax in frequenz-floss/frequenz-dispatch-python#54 (comment)

[llucax]

This came up again, as there seems to be easy to get confused by the different meaning of duration based on if it is 0 or None, so it might be a good idea to prioritize it @Marenz.