Extensibility of the API

[m-mohr]

Currently, we have the core API, which covers everything we need. I'm wondering whether it may be a good idea to keep it a little smaller and allow extensions to it. For example

• the subscriptions API,
• the sharing functionality (Sharing resources #85),
• WFS/STAC Items/Tiles extension (basically /collections/{collectionID}/items...)
• UDFs (Language-agnostic UDFs #87) and
• OpenSearch (OpenSearch support for data discovery #68)

could live in its own specifications as extensions. We would need to define a procedure for extensions and add something to list them, something like /conformance in WFS or directly in the / endpoint.

I think this is a good idea, to keep the Core API smaller and allow extensions like the subscriptions API, udf, open search and especially the sharing functionality.

[m-mohr]

We have conformance, stac_extensions and the endpoints list in GET / for extensibility, which we can re-use. So for now I'll not add anything new here and just see how it goes with those structures.