Ensure cql2-text is supported for GET filter search requests

[jonhealy1]

• Ensure cql2-text is supported for GET /search and GET /collections/{collection_id}/items endpoints.
• According to this, stac-fastapi-pgstac supports cql2-text but not sfeos.
• cql2-text makes using the filter extension easier for GET requests versus only supporting cql2-json

[jonhealy1]

Seems to work fine ...

1. Write some more tests for this functionality
2. Make a pr at the link above to change the information that is provided - stating that we don't support cql2-text