[Feature] Add option to insert additional using statements for the entity

[ArnaudB88]

In our company, we use your generator for many years now with many developers. Over the years, we added some features so it works better with our solutions. As a sign of appreciation, I want to suggest some features because I think it will help some other developers as well.

Feature description:

Add an option to insert additional using statements in the generated entity classes.

Feature motivation & example usage:

This can help making the generated classes cleaner when you also add base classes, interfaces, attributes (see other feature request #547 ).
We use it to add the namespace of our added interface in addition.
The option is a ; separated string, so multiple using statements can be added.

YAML usage:

data:
  entity:
    baseClass: 'IEntityBase'
    additionalUsings: 'Company.Project.Domain.Interfaces'

Code generation result:

using System;
using System.Collections.Generic;
using Company.Project.Domain.Interfaces;

namespace Company.Project.Domain.Entities;

public partial class Project
    : IEntityBase
{
}

Please review the feature request and evaluate if you think this is an addition to your repo.
(pull request will be added)

Thanks