STAC catalog and collection

Currently we only generate STAC Items, and a lot more can be achieved with adding Catalog+Collection. This is somehow also tied to the general data model which we define in our global values. Assuming a relatively simple and flat hierarchy (no weird collection in item or catalogue in collection) we should have the info to generate a catalog.json and necessary collection.json files. The main question is if this should only happen in the preprocessor, and ties to the question on general STAC metadata storage.

Catalog - relates to a single VS deployment
|
|
- Collection_1 - self explainatory
  |
  - Item_1 - product with assets as coverages
  - Item_1
- Collection_2
- ...

Related, because of linkage additions: #7 (closed), preprocessor-v3#5 (closed)