Add function that is called before saving a model

# Feature request
Add a function which is called before saving.

### Context
I want to implement a updated_at field that contains the Date/Time of the last update. This can easily be implemented with such a function. Other use cases include a field that is dependent on another field eg.: file_extention = "pdf" is dependent on file_path = "/home/Documents/test.pdf"

### Solution

Add a "pre_save()" function in the "Model" class that can be implemented by Models. Call this function from the "engine._save()".

#### Alternative solutions

Calling a function every "save" manually.

### Additional context

If added, this has also be added to the documentation.

Start by reading the Model class and the engine._save() entry point named in the issue to understand the current save flow. Check the existing documentation and tests around model saving before deciding what coverage is needed. Done means a pre_save() hook is supported for models, is called before saving, and the behavior is documented.

Written by the indexing model from the issue text.