feat: add stapi-pydantic

[gadomski]

I copied the files from https://github.com/stapi-spec/stapi-fastapi/tree/main/src/stapi_fastapi/models and https://github.com/stapi-spec/stapi-fastapi/tree/main/src/stapi_fastapi/types and updated them to work. A couple of other choices and comments:

• I switched to https://pypi.org/project/cql2/ as it's lighter than pygeofilter
• I removed the callables from the pydantic models, they'll need to get re-added in a subclass (or something) in stapi-fastapi
• I removed the distinction between "models" and "types"
• Needs moar tests