Validate that items have a `collection` link if they have `collection` set when decoding

[jisantuc]

Describe the bug
Items that set the collection property must also have a link with the collection rel type (spec). We have enough info to validate that at decoding time, so we should do that.

Expected behavior
Invalid items can't be decoded and fail with an appropriate message.

Additional context
https://azavea.slack.com/archives/CM766K2QJ/p1627420112000800