PoC for a ProcessorMixin class
# What does this PR do?

This PR refactors some common saving/loading functionality of `Processor`s behind a `ProcessorMixin` class which implements the `save_pretrained` and `from_pretrained` method.

It's designed to work with main attributes (by default `feature_extractor` and `tokenizer`, but that list can be changed or extended in subclasses) that are then required by the init and on which `save_pretrained`/`from_pretrained` is called when saving or loading the processor.

It also handles having several classes for a given tokenizer (fast/not fast) so we can have that feature automatically handled. I showed how this works in refactoring two processors (I did not put the tokenizer fast in CLIP because there are some problems with it, but adding it is as easy as changing the `tokenizer_class`).
I will refactor the other processors if the design suits everyone.
_The documentation is not available anymore as the PR was closed or merged._