roles in common metadata

[m-mohr]

We are using roles in Providers and Assets right now.

I think it would be useful to make roles available in common metadata so that we can assign roles to other entities, e.g.

• bands (e.g. to identify a band as spectral, clouds, nodata mask, ...)
• links (rel is a single string and sometimes it would be useful to assign multiple values, that are also more freely usable compared to the values that are often pre-defined by IANA/OGC).
• Contacts extension
• ...

Thoughts?