Extract Landsat backend into new package/repo

[jkeifer]

stapi-spec/stapi-fastapi#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.

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