fix(api-model): correct AllOptionalMetaclass for field validation in form models

[laurent-laporte-pro]

The goal of this PR is to correct the implementation of the AllOptionalMetaclass metaclass used to implement Pydantic models for reading and updating configuration data (short-term storage, thermal clusters, renewable resources clusters, etc.).

The metaclass has been fixed to allow field validation when they come from an ancestor class to the model (using inheritance).

The use_none option has been added to the metaclass parameters to indicate that we want to use the None value instead of the default values. This is necessary, for example, to update certain fields of a model while leaving other fields unchanged: the None value is used to indicate the fields that we wants to preserve.

Unit tests have been corrected to take into consideration the form field validation (previously, validation was not occurring).

Minor changes have been made to correct the documentation and reorganize/fix module imports.