Add support for COCO formats

[sfmig]

Is your feature request related to a problem? Please describe.
We would like to support a wide range of formats.

Describe the solution you'd like
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Describe alternatives you've considered
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Additional context
References for COCO formats: see here and here

[adamltyson]

FYI - talmolab/sleap-io#102

[talmo]

Thanks for the crosslink @adamltyson!

One issue I can foresee here is that there are many different variants of the COCO format with slight nuances in how they differ.

I'll also add that there tends not to be as many that are oriented towards storing predictions, but rather the ground truth data.

For example: mmpose animal datasets: directory structure is all over the place, no identities provided (most datasets are not contiguous frames). [Dataset format info].(https://mmpose.readthedocs.io/en/latest/user_guides/dataset_tools.html).

If there are specific datasets you guys had in mind to support, we can make sure that our implementation is compatible.