✨ Expose validation error fields and add error mapping utilities

[flovouin]

📝 Description of the PR

Adds a fields property to the ValidationError, listing the path to each property that failed validation. Nested objects and arrays are referenced using their full path, e.g. child.id or items.0.name. The property is populated by the validateObject and parseObject utilities, and is empty when the validation failed for the payload as a whole.

Several utilities mapping business errors to their ErrorDto counterpart are also introduced, meant to be used with the tryMap utility and the @TryMap decorator: incorrectEntityVersionErrorAsDto, validationErrorAsDto, and forbiddenErrorAsDto. validationErrorAsDto formats the validation messages as a bullet list and returns the failing fields to the caller. It optionally accepts an error type, such that a subclass of the ValidationError can be matched instead of the base error.

The forbiddenErrorAsDto utility maps the new ForbiddenError, a business-level error meant to be thrown by authorization logic. Because such logic is usually, but not always, used in an HTTP context, the error itself is defined independently of NestJS. Its message is not returned to the caller, such that it can freely describe why the operation was denied.

📋 Check list

• 🧪 Unit tests have been written.
• 📝 Documentation has been updated.