Improve JSON Schema

[m-mohr]

Update JSON Schema:

• Include bands as e.g. in https://github.com/stac-extensions/accuracy/blob/main/json-schema/schema.json
• Simplify, maybe even use if/then/else to make validation less verbose

[m-mohr]

A new version that better checks all parts and properly checks the required fields:
https://github.com/stac-extensions/attribution/blob/main/json-schema/schema.json