Configurable resampling interpolator

[joeranbosma]

Allow setting the resampling interpolator for preprocessing.

Dependency updates:

• requirements_dev.txt: Updated versions of flake8, tox, pytest, pytest-cov, mypy, build, and twine.
• setup.py: Incremented the version number from 2.1.8 to 2.1.9.

Interpolation settings:

• src/picai_prep/preprocessing.py: Added scan_interpolator and lbl_interpolator attributes to the PreprocessingSettings class.
• src/picai_prep/preprocessing.py: Modified the resample_img function to accept an interpolation parameter and use it if provided. [1] [2]
• src/picai_prep/preprocessing.py: Updated the resample_to_first_scan and resample_spacing methods to use the new interpolation settings from PreprocessingSettings. [1] [2]