Use a 'via' rel to link back to the Planet data API url that is equivalent.

[cholmes]

For RC1 release we will add a 'via' rel, see https://github.com/radiantearth/stac-spec/blob/dev/best-practices.md#using-relation-types

This is perfect to point back at the URL of the data API of Planet, as the original, online source of the STAC record. We can use the 'via' without needing the new STAC release, as you can put anything in 'rel' links.

So for each Item we should construct the data API url from the item_type and ID, and put it in the links with rel = via.