[C#] DateTimeFormat not used in objects serialization and deserialization by Newtonsoft Json.Net

Description

DateTimeFormat is not managed in the JsonConvert serialization and deserialization.
ParameterToString will correctly use it (see #1706) but for complex objects this settings would not be used.

Swagger-codegen version

swagger-codegen-cli-2.4.8 and online version (https://editor.swagger.io/)

Suggest a fix/enhancement

Two proposed fix in ApiClient.cs:

1. Constructor by configuration
   ` public ApiClient(Configuration config)
   {
   Configuration = config ?? Client.Configuration.Default;

        // here we probably would use Configuration.DateTimeFormat if it's valorized also in serializerSettings for Newtonsoft JsonConvert
        if (string.IsNullOrEmpty(Configuration.DateTimeFormat))
        {
            serializerSettings.DateFormatString = Configuration.DateTimeFormat;
        }
   
        RestClient = new RestClient(Configuration.BasePath);
    }`
   

2. The Serialize part
   public String Serialize(object obj) { try { return obj != null ? JsonConvert.SerializeObject(obj, serializerSettings /* here we need to pass the serialization settings*/) : null; } catch (Exception e) { throw new ApiException(500, e.Message); } }

Start in the generated C# ApiClient.cs, reading the serializerSettings setup, Serialize method, and the existing ParameterToString handling referenced in #1706. Trace both object serialization and deserialization, then verify that Configuration.DateTimeFormat is honored consistently; the issue does not name a test, so identify the relevant test location before changing behavior.