Extract Landsat backend into new package/repo

[jkeifer]

#79 removed the Landsat backend. We'd like to extract that backend from the git history and make it into its own python package in a new repo in this organization. That implementation should be extended to support ephemeral ordering via an in-memory orders ring buffer, and orders should be fulfilled via the Landsat collection from Earth Search.

Last commit with Landsat backend: 8899ad6
Earth Search Landsat collection: https://earth-search.aws.element84.com/v1/collections/landsat-c2-l2