Closes #545 Add option for additional using statements in the entity

[ArnaudB88]

Resolves #545
Add an option to insert additional using statements in the generated entity classes.

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
{
}