Invalid schema for STAC Collection

[fmigneault]

The README seems to indicate that this extensions applies to STAC Items and Collections.
The use cases documentation https://github.com/stac-extensions/ml-aoi/blob/main/docs/0003-test-train-validation-split.md also suggests some valid use of the split indication at the collection level.

I indeed find it really relevant to provide ml-aoi:split: train|validate|test on the collection that aggregates all STAC Items with the corresponding splits.

However, the schema currently only supports STAC Items (and explicitly rejects Collections) with:

ml-aoi/json-schema/schema.json

Lines 16 to 19 in 4aa0c49

	"properties": {
	"type": {
	"const": "Feature"
	},

The schema should be more permissive to allow the STAC Collection use case.