Option for CEOS ARD compliant generation

[m-mohr]

Provide an option to generate STAC metadata that is compliant with the CEOS ARD STAC extension:
https://github.com/stac-extensions/ceos-ard/blob/main/optical.md

I'll try to figure out in this issue what is missing from the package to support this.

Collection:

• ceos_ard:type = "optical"
• ceos_ard:specification = "SR"
• ceos_ard:specification_version = "5.0.1"
• 3.4/3.5: add sci:doi and/or link to https://doi.org/10.5270/S2_-znk9xsj
• optionally, add https://sentinels.copernicus.eu/web/sentinel/technical-guides/sentinel-2-msi/level-2a/algorithm-overview

Item:

• Add classification extension or similar for SCL (2.x requirements):
  

Example: https://radiantearth.github.io/stac-browser/#/external/raw.githubusercontent.com/stac-extensions/ceos-ard/main/examples/optical-sr/collection.json

I'll work on a PR.

[m-mohr]

For Items: #175

It seems no Collections get generated in this package, so irrelevant for now.