external enhanced product types metadata

[sbrunato]

Implement a mechanism to allow extending product types metadata using external json files.

Optionally add per product type in product_types.yml the path to a json file (local or remote).
Use search.discover_product_types.generic_product_type_parsable_metadata from eodag/resources/stac_provider.yml to extract product types properties in client mode.
In server mode, use client mode product types properties, overridden with external json file content.

[alambare]

@sbrunato Setting jsonpath per metadata sounds very complicated and very cumbersome. Maybe we could go with a more simple and straightforward implementation.

[sbrunato]

@sbrunato Setting jsonpath per metadata sounds very complicated and very cumbersome. Maybe we could go with a more simple and straightforward implementation.

description updated!