add 'acknowledgement' field to STAC Item and Collection

[J-Christophe]

• Add optional 'acknowledgement' field at root level in STAC Item and Collection.
• Add 'acknowledgement' field in 'sci:publications' objects.
• Add 'acknowledgement' field in 'assets' and 'item_assets' for granular recognition.
• Update README and examples to include acknowledgements for clarity and consistency.

[J-Christophe]

@m-mohr Could have a look please ?

[m-mohr]

Why shoukd acknowledgements be in the STAC metadata? It's likely not searched on and usually for human consumption and as such also be part of the description.

[J-Christophe]

In many scientific contexts, acknowledgements are a distinct, formal element of dataset documentation. They often include funding agencies, project identifiers, or institutional collaborations, which are:

• mandatory in several research and space agency standards (e.g., ESA, NASA, CNES, Horizon Europe);
• useful for automatic metadata export or mapping to standards like DataCite, Dublin Core, or ISO 19115 (where similar fields exist, e.g. credit or fundingReference);
• and important for traceability and FAIR compliance.

The field would remain optional and purely informational, but separating it from description improves clarity and future interoperability — especially for systems that automatically generate or transform metadata.

[J-Christophe]

@m-mohr Are you convinced ?