Disambiguate the use of `ml-aoi:role` and Asset `roles`

[fmigneault]

When using ml-aoi:role under a Link using rel: derived_from (see https://github.com/stac-extensions/ml-aoi?tab=readme-ov-file#links), the specific ML-AOI role makes sense as there is no "role" otherwise provided. However, using ml-aoi:role under an Asset seems somewhat redundant, since Assets already contains a roles property. The ml-aoi:role under an Asset remains valid to be more specific than the properties at the STAC Item level (which would apply to all underlying Assets), but the specific use of the term "role" can lead to confusion.

Should there be some additional requirements that add define additional ML-AOI specific roles under the Asset Roles directly? Would that cause even more confusion? Is the solution just to modify the term "role" globally?

For example:

assets:
  label:
    ml-aoi:role: label  # this to be removed?
    roles: 
      - ml-aoi:label  # or simply "label"?
      - data
  raster:
    ml-aoi:role: feature # this to be removed?
    roles: 
      - ml-aoi:feature # or simply "feature"?
      - data
      - source

[m-mohr]

Reusing the roles (which will become part of common metadata in STAC 1.1, Ithink) would be good. We prefixed roles in other extensions, too.