Stac-pydantic

- Bugfixes (10)
- Add rel types enum (11)

- Add models for the STAC API spec (7)

- Allow extra asset-level fields (1)
- Fix population by field name model config, allowing model creation without extension namespaces (2)
- Add enum of commonly used asset media types (3)
- Move geojson models to `geojson-pydantic` library (4)