Implement STAC ml-model extension

[ymoisan]

Describe DL model characteristics -- e.g. metadata like what data we used for training, feature types that can be extracted -- using the emerging STAC ml-model extension.

• [ ] Help define a stable/usable (tagged) version of ml-model [work with the community]
• [ ] Implement STAC model item generation in GDL following a specific version of ml-model

Once we have that we can think of implementing business logic to suggest models to a user as a function of image and features wanted for extraction.