add standard existing extensions not included in pystac

pystac already contains several extensions: https://pystac.readthedocs.io/en/stable/api/extensions.html

However, numbers of extensions are defined but not coded as a pystac extension, see STAC extensions grouped by maturity

Most used extensions could be included in pystac. Example:

• sentinel-1
• sentinel-2
• processing
• classification

The common_metadata, that is actually an aggregate of schemas, may be a special case, as on the main STAC platforms (planetary-computer, element84, ...) the schemas of these metadata are not specified...