Support field mapping in STAC

[mhogeweg]

in STAC support field mapping from field names used in STAC items (e.g. eo:cloudcover) to index-specific field names (e.g. eo_cloudcover_f = float) to make use of the index aggregations. simple BNF-style definition:

      field_mapping ::= stac_field_name=index_field_name
      stac_field_name=index_field_name ::= name + [_txt | _s | _b | _i | _l | _f | _d | _dt | _cat ]
      _txt ::= free text field
      _s ::= keyword field
      _b ::= boolean field
      _i ::= integer field
      _l ::= long field
      _f ::= float field
      _d ::= double field
      _dt ::= date field: e.g. 2025-01-16T11:35:56.732Z
      _cat ::= hierarchical category field: e.g. continent > country > state/province > county/municipality > city > neighborhood ...