Label Examples need to be Updated

[m-mohr]

Origin: radiantearth/stac-spec#870 @notthatbreezy

Some of the examples in the label extension are outdated and could use updating.

For instance, in the spacenet roads example label:assets should be an array. The item linked to and the asset key referenced in the example are not actually the source that the labels apply to either.

Similar issues apply to the other examples in the label extension as well.

[m-mohr]

I've updated the examples to the recent STAC version and fixed some structural issues with the stac_extensions field, but other issues mentioned above have not been tackled yet.

[m-mohr]

I added JSON Schema validator for label:assets in links and made the label:assets field an array in all examples, but I did not further verify that the values make sense ("the asset key referenced in the example are not actually the source that the labels apply to either."). So still some work to do here.