Refactor NDVI process and update dependencies

- Removed geoengine-datatypes and geoengine-operators dependencies from Cargo.toml.
- Updated NDVI process in ndvi.rs to use new VectorOperator and RasterOperator structures.
- Refactored source functions to return RasterOperator directly instead of boxed trait objects.
- Modified to_api_workflow function to accept VectorOperator and adjusted workflow type handling.
- Updated unit tests to reflect changes in operator structure and ensure correct serialization.

Author: ChristianBeilschmidt