Request: CreateML JSON format for Action Classifier

I was playing around with the "Video Frame Classification" feature of label studio to see if I could introduce it into my workflow. Right now the annotation mechanism is decent, but the export file feature doesn't seem to export something that could be converted easily into Apple's CreateML annotation json forma for Action Classification.

CreateML Action Classification format is relatively straight forward ([source](https://developer.apple.com/documentation/createml/building-an-action-classifier-data-source#Build-a-Data-Source-with-Annotated-Videos)), but it primarily uses one label per line or dict, with a start and end time-stamp of the video (not frames).

CSV
```
video, label, start, end
,,,
```

JSON
```
[
{
"video" : "filename as string.mov",
"label" : "label as string",
"start" : ,
"end" :
}
]
```

Time indexes can be either:
- An integer of seconds, for example: 0, 3, or 5.
- A floating point number of seconds, for example: 0.0, 3.14, or 60.5.
- A string of minutes and seconds, for example: 01:03.
- A string of minutes, seconds, and fractional seconds, for example: 01:03.14.
- A string of hours, minutes, and seconds, for example: 05:01:03.

Hope this is something that could be considered in the feature cycle. Thank you.