fix filter extension implementation

[vincentsarago]

closes #148

This PR fixes the CoreCrudClient.item_collection and CoreCrudClient.get_search method implementation for the filter extension .... and add tests ;-)

cc @hrodmn

[hrodmn]

@vincentsarago thanks for adding these changes! Right now stac-fastapi does not support filter-lang=cql2-text in POST requests. That is not required per the spec but is also not obvious to API clients. Fortunately if you try to send a cql2-text string in the filter parameter it will be rejected since it does not match the type expectation set by FilterExtensionPostRequest.

Would it make sense to allow either cql2-text or cql2-json (or cql-json) in POST requests?

[vincentsarago]

oh good point @hrodmn

I guess we need to change the type definition here https://github.com/stac-utils/stac-fastapi/blob/main/stac_fastapi/extensions/stac_fastapi/extensions/core/filter/request.py#L49

note: We should also add tests