stac subtype: Include ItemCollections?

[m-mohr]

The stac subtype (see https://github.com/Open-EO/openeo-processes/blob/draft/meta/subtype-schemas.json#L281) currently allows to pass a single item, collection or catalog (which includes API landing pages). It does NOT include ItemCollections, which is what you get returned by a STAC search (i.e. a GeoJSON FeatureCollection where each feature is a STAC Item).

Is that good or bad?

Came up in a discussion here: opengeospatial/ogcapi-processes#467

First thought was that it might be good to add ItemCollection, but then on the other hand I'm thinking that it may be bad because we actually may want to do the search implcitly using the parameters provided in the load_* processes.

So just capturing some thoughts from otheres here that implement it.

cc @clausmichele @soxofaan @jdries