CQL 2 Support

[bitner]

• Update CQL2-JSON parser to current spec
• Add CQL2-TEXT parser
• Adds a backend to output CQL2-JSON (Use case for this - pgstac only supports cql2-json, so this allows middleware (ie stac-fastapi) to convert any pygeofilter supported input)
• Removes Python 3.6 from CI due to issue caused by error due to lack of datetime.fromisoformat() in 3.6
• Adds tests that make sure that ASTs generated between examples on the spec for CQL2-TEXT and CQL2-JSON as well as output generated by the CQL2-JSON backend are equal.

[bitner]

@tomkralidis @constantinius let me know if there is anything that I can add here to help y'all review this

[constantinius]

@bitner Awesome, thanks a lot for your contribution.

I see the CQL2-Text implementation is also included, based on https://github.com/geopython/pygeofilter/tree/cql-text I guess. This branch was not yet ready, and the text parsing not really functional. Should we already include it?

[bitner]

@constantinius I used that branch as my starting point, but there are substantial changes and things that were fixed up. The cql2-text in this PR should hopefully be fully or at least mostly functional.

[constantinius]

@bitner Alright, thanks again!