Remove dependency between Aggregate and Filter extensions

[vincentsarago]

@jamesfisher-gis sorry I'm just realizing this now but I don't think we should do this, at least I don't see in the aggregate extension why the filter attributes should be enabled with the aggregations ones.

I think it will be up to the implementers to add the filter extension and also handle them in the client.

Originally posted by @vincentsarago in #684 (comment)

[vincentsarago]

stac-fastapi/stac_fastapi/extensions/stac_fastapi/extensions/core/aggregation/request.py

Line 24 in 8f400e1

aggregations: Optional[Union[str, List[str]]] = attr.ib(default=None)

also realizing that in ☝️ we use attr.ib instead of pydantic.Field

[jamesfisher-geo]

Hey. That makes sense.

I have a couple other small bug fixes for aggregation. I will submit a PR today that removes the Filter extension dependency.