Use Landsat extension instead of Planet specific fields?

[m-mohr]

STAC has a specific Landsat extension, as such the corresponding fields should be used instead of Planet proprietary ones.

Also, the pl:wrs_* and pl:collection fields are not defined in the STAC extension, but are used in at least one example:
https://github.com/planetlabs/stac-extension/blob/main/examples/items/Landsat8L1G.json

Unfortunately, the landsat extension requires a large number of fields, so we could use them, but not add the schema to stac_extensions (or add more fields to the JSON).