Link rel type for public sharing of UDP

[soxofaan]

I'm currently working on a proof of concept implementation of public sharing of UDPs.
One of building blocks is how the owner of the UDP can obtain a publicly sharable URL (e.g. signed URL) pointing to the UDP metadata (including process graph).
The easiest solution is probably just adding a link to the "links" field of the UDP's metadata at /process_graphs/{process_graph_id}.

Standardizing UDP sharing as a whole will probably be a big task, but I think it's already possible to settle on the rel type of that link.

I couldn't find a suitable one in the list of standardized link relations referenced from https://openeo.org/documentation/1.0/developers/api/reference.html#section/API-Principles/Web-Linking .
At the moment I'm using "rel": "public", but I'm open for better suggestions.

[m-mohr]

Interesting topic, indeed. There's definitely room for alignment.

What seems most logical here is the relation type canonical (which means this is a publicly available permanent link to the resource), which we also use in the batch job results for a similar purpose, see #400.

I'll draft a PR for this.

[m-mohr]

See the commit above, which makes the canonical relation type apply generally and not just to jobs. So UDP sharing or batch jobs are a use-case here.

Please let me know if there's still room for improvement.