Implement json encoding for records response

[pvgenuchten]

https://github.com/opengeospatial/ogcapi-records/blob/338e959f7d60d89a05c0bdf50003df6051116dd6/core/openapi/schemas/record.yaml defines a typical ogc-api records response encoded as json (or xml). Would be good to be able to return this format from the elastic index (without transforming any xml).

[fxprunayre]

The main issue here is that we are going to have a variety of JSON formats and the end user may have difficulties to find which one to use :)

• Index doc in ES
• GeoJSON ?
• OGC API Records JSON
• schema.org as JSON-LD.