remove FieldsExtension dependency in `stac-fastapi.api`

[vincentsarago]

in 👇 https://github.com/stac-utils/stac-fastapi/blob/51a756d904e4bab245da4a525e961d64bcbf1321/stac_fastapi/api/stac_fastapi/api/app.py#L255-L263

We check if the FieldsExtension extension is part of the allowed extension, and then remove output model validation if present (because when using fields, the output Item can be an invalid STAC Item).

This works OK, but brings multiple issues:

• What if no fields parameter is passed and the application is set to validate the items?
• it makes stac-fastapi.api depends on stac-fastapi.extensions

IMO, this is an implementation issue that stac-fastapi.api shouldn't take care.

At the implementation level should could be dealt like:

class BadCoreClient(BaseCoreClient):
    def post_search(
        self, search_request: BaseSearchPostRequest, **kwargs
    ) -> stac.ItemCollection:

        search_results = ...

        if getattr("fields", search_request):
            return GeoJSONResponse(search_results)

        return stac.ItemCollection(**search_results)

    def get_search(
        self,
        collections: Optional[List[str]] = None,
        ids: Optional[List[str]] = None,
        bbox: Optional[List[NumType]] = None,
        intersects: Optional[str] = None,
        datetime: Optional[Union[str, datetime]] = None,
        limit: Optional[int] = 10,
        **kwargs,
    ) -> stac.ItemCollection:

        search_results = ...

        if kwargs.get("fields"):
            return GeoJSONResponse(search_results)

        return stac.ItemCollection(**search_results)

☝️ This pseudo code show that when a fields parameter is passed we should return the search_results directly by returning a Response object, which won't be validated by fastAPI

[jonhealy1]

@vincentsarago How should we move ahead with this?

[vincentsarago]

@jonhealy1 it's a big breaking change IMO, and we should wait for 4.0 for this kind of change.

[vincentsarago]

I think I changed my mind and we should ship this in 3.0 🙈