backend implementation for queryables in the labextension

[alambare]

EODAG recent update introduced the list_queryables() method. This method exposes a list of known filters for users helping them to create valid requests for their preferred provider and product_type.

This change is the first part to bring this improvement to the labextension.

We need a API endpoint that can be called by the front to retrieve the queryables for a specific provider and product_type.

The queryables endpoint must return a json schema containing all the queryables in a properties key. A required property with a list of required properties' name as values. The additionalProperties bool to specify whether additional properties are allowed.

We can retrieve the json schema using pydantic like the following

queryables_dict = dag.list_queryables(provider="cop_ads", productType="CAMS_GAC_FORECAST")
json_schema = queryables_dict.get_model().model_json_schema()

The None default values of properties should me removed as they make ugly json schema with pydantic.
See how this is done in the server mode: https://github.com/CS-SI/eodag/blob/develop/eodag/rest/core.py#L619-L620