Time trackers

[Brezak]

Objective

Users currently need to update timers and stopwatches themselves. This introduces 2 main issues:

• Users may forget to update their timers.
• Timers read from both Update and FixedUpdate need special considerations when updating them

Solution

• Introduce 2 new component versions of timers and stopwatches.
  • Updating* time trackers track time based on their generic argument (e.g. UpdatingTimer<Virtual> will track Time<Virtual>).
  • Mixed* time trackers track both virtual and fixed time and correctly report time based on which schedule they're read from.
• Introduce a TimeTracker trait defining how to update a time tracker. The intent is to eventually provide a derive macro.
• Introduce a Context trait to distinguish Time<()> from all the time types. This is used to constrain what time type the updating time trackers track.
• Introduce a TimesWithContext to define what times a TimeTracker tracks.
• Introduce a TimeTrackingAppExtension trait that helps with registering user defined time trackers into the app.

The `TimeTracker` trait

pub trait TimeTracker {
    type Time: TimesWithContext;

    fn update(
        &mut self,
        time: &<<Self::Time as TimesWithContext>::AsSystemParam<'_> as SystemParam>::Item<'_, '_>,
    );

    fn enter_fixed_update(&mut self) {}
    fn exit_fixed_update(&mut self) {}
}

Closes #13420.

Future work

• A TimeTracker derive macro.
• Automatic cleanup of finished time trackers.

Testing

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• Are there any parts that need more testing?
• How can other people (reviewers) test your changes? Is there anything specific they need to know?
• If relevant, what platforms did you test these changes on, and are there any important ones you can't test?

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