Feature request: Style root level property "attribution"

[pathmapper]

Motivation

Sometimes it is required to attribute a style different than the sources used in the style.

Design Alternatives

• Currently it's possible to use a dummy source to pass such a style attribution to the AttributionControl (Support custom attribution strings #1485 (comment)), so do nothing is an option.

• If Support custom attribution strings #1485 would be implemented it would be possible to achieve this with GL JS.

• Implement an optional style root level property attribution, which (if present) is passed to the AttributionControl.

Design

Which design should we implement?
I favor a style root level property attribution.

What are the advantages of this design?
No need to use dummy sources (and dummy layers after #6281 is fixed).
It's possible to included the proper attribution in the style.
No need to pass a custom attribution string to the AttributionControl (if possible after #1485 is implemented).

What are some potential drawbacks of this design?
It's an addition to the style spec. Although such a property would be optional, maybe it affects also other implementers of the spec.