Make a base init in FeatureExtractionMixin
# What does this PR do?

This PR adds a base init to `FeatureExtractionMixin` so that high up in the hierarchy, remaining kwargs are set as attributes. Without this, adding new fields to the config of a feature extractor inheriting directly from `FeatureExtractionMixin`, like `ViTFeatureExtractor`, will fail so we won't be able to add fields (like the model type for easy use with an Auto API) without breaking backward compatibility.