Allow me to declare an animation clip event based on a frame, not on a time

## What problem does this solve or what need does it fill?
The fact that i can only declare an animation event based on a time and not a frame is not only weird, but it is confusing and potentially even makes it impossible for me to do what i need to do (i am not sure?)

My running animation has a dynamic speed. So i need to be able to trigger events at particular frame indices. 'time' will not work if it is literally using time with no regard for the speed of the clip ( i dont know as this is not documented).

So what am i to do here? IMO either the documentation needs to be very very much improved in this regard, or (my primary recommendation) make these anim clip events be based on a frame index in order to make this clear and impossible to get wrong or mess up .

Example:
```

anim_clip.add_event_to_target(
AnimationTargetId::from_iter( bone_names_array ),
clip_time.clone(), // makes no sense for this to me a time, it should be a frame index
anim_clip_event_type.clone()
);

```

Start with the anim_clip.add_event_to_target entry point shown in the issue and inspect how its clip_time argument is interpreted. Determine whether the requested behavior requires frame-based events, clearer documentation of time semantics, or both; done means the chosen behavior is explicit and no longer ambiguous for dynamically paced animations.

Written by the indexing model from the issue text.