Add version STAC extension metadata for collections

[tylere]

Earth-search v1 currently contains both current (sentinel-2-c1-l2a) and deprecated (sentinel-2-l2a) collections. To facilitate steering potential users to the current collection, it would be helpfull to add Version STAC Extension metadata. This could help to avoid questions/issues such as #34.

Specifically, it would be helpful to have the sentinel-2-l2a Collection include

"deprecated": true

and a link object that points to the sentinel-2-c1-l2a Collection

{
  "rel": "latest-version",
  "href": "https://earth-search.aws.element84.com/v1/collections/sentinel-2-c1-l2a"
}

[matthewhanson]

@tylere thanks for the suggestion, it's a good one. We've been avoiding officially deprecating the collection because the new one still isn't complete as ESA is still doing backprocessing, but it's probably complete enough that we should be pointing folks to the new collection.