consolidate STAC configuration json files

[mhogeweg]

minimize what needs to be configured (stac-item.json/stac-items.json). research loading STAC property definitions from JSON schema file. See https://stac-extensions.github.io/ and https://github.com/stac-extensions/sentinel-2/blob/main/json-schema/schema.json.

consider separate utility 'prepare STAC configuration'

option to generate mapping from for example s2:product_uri (a string), to s2_product_uri_s