Update AuthController and Login model for API changes

- Updated API documentation in AuthController.cs to include an optional "id" field in the authentication request body.
- Modified the Login record in Login.cs to require a "Password" field and include optional "Id" and "Username" fields, along with updated documentation comments.

EnvelopeGenerator.GeneratorAPI/Controllers/AuthController.cs

@@ -51,6 +51,12 @@ namespace EnvelopeGenerator.GeneratorAPI.Controllers
         ///        "username": "MaxMustermann",
         ///        "password": "Geheim123!"
         ///     }
+        ///     
+        ///     POST /api/auth?cookie=true
+        ///     {
+        ///        "id": "1",
+        ///        "password": "Geheim123!"
+        ///     }
         ///
         /// </remarks>
         /// <response code="200">Erfolgreiche Anmeldung. Gibt das JWT-Token im Antwortkörper oder als Cookie zurück, wenn 'cookie' wahr ist.</response>