Support STAC ItemCollection in `load_stac`

[soxofaan]

Side question from #523 (comment) that deserves it's own ticket/discussion I think:

related question brought up at Open-EO/openeo-geopyspark-driver#1278:
should load_stac also support loading STAC ItemCollections?

so technically: should it support urls pointing to a STAC ItemCollection (e.g. a "deep" URL encoding a STAC API GET query)?

[soxofaan]

at the moment, only these documents are supported

openeo-processes/proposals/load_stac.json

Line 13 in 98ae014

"description": "The URL to a static STAC catalog (STAC Item, STAC Collection, or STAC Catalog) or a specific STAC API Collection that allows to filter items and to download assets. This includes batch job results, which itself are compliant to STAC. For external URLs, authentication details such as API keys or tokens may need to be included in the URL.\n\nBatch job results can be specified in two ways:\n\n- For Batch job results at the same back-end, a URL pointing to the corresponding batch job results endpoint should be provided. The URL usually ends with `/jobs/{id}/results` and `{id}` is the corresponding batch job ID.\n- For external results, a signed URL must be provided. Not all back-ends support signed URLs, which are provided as a link with the link relation `canonical` in the batch job result metadata.",

The URL to a static STAC catalog (STAC Item, STAC Collection, or STAC Catalog) or a specific STAC API Collection that allows to filter items and to download assets. ...

[m-mohr]

Related: #523

[m-mohr]

This is an addition to the load_stac to allow for STAC APIs additionally passing in a Global Item Search or Collection Items url with pre-defined query parameters. The main question is how to resolve conflicting parameters (e.g. bbox in the URL and bbox provided in the process parameters). Discussion today leans towards applying the parameters in the process in addition (which effectively is the intersection).