Implement filter extension fot GET /search

@philvarner wrote code to implement the filter extension for GET requests but it was commented out as we were waiting for changes to be made to stac-fastapi. https://github.com/stac-utils/stac-fastapi-elasticsearch/blob/bd55adef34153ac7cbbdda62456f923e1d5d40cb/stac_fastapi/elasticsearch/stac_fastapi/elasticsearch/core.py#L342-L347