Export function signatures for user

[aaarrti]

Description of the problem

• Quantus is designed with a goal of being highly customizable. At this time, however, if user want to define his own, e.g., similarity_func, they need to read into the source code.

Description of a solution

• Define type aliases for commonly used functions, e.g, SimilarityFn = Callable[[np.ndarray, np.ndarray], np.ndarray]
• Export type aliases at top level of library
• As a bonus, we could have more precise type hints in metrics, without overwhelmingly long type annotations.
• Ideally, could be done together with https://github.com/understandable-machine-intelligence-lab/Quantus/issues/70