Bug: custom serializer settings are ignored

When setting custom serialization settings on JsonApiOptions, they are not used. In our scenario we have a few enums exposed as attributes and we want them to serialize into their text representation instead of numeric.

Example:

public enum ScreenOrientation
{
    Horizontal, Vertical
}

public class DemoResource : Identifyable
{
    [Attr]
    public ScreenOrientation Orientation { get; set; }
}

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddJsonApi<AppDbContext>(options =>
        {
            options.SerializerSettings.Converters = new List<JsonConverter>
            {
                new StringEnumConverter()
            };
        });
    }
}

Putting [JsonConverter(typeof(StringEnumConverter))] on ScreenOrientation works, but the global setting does not.

Using the global setting should work, according to https://github.com/json-api-dotnet/JsonApiDotNetCore/blob/master/docs/usage/options.md#custom-serializer-settings.

[bart-degreed]

Update: After discussion, we decided to keep the Newtonsoft.Json setting and remove ISerializerOptions.

[bart-degreed]

To clarify: The work that needs to be done here, in additional to the changes in #688, is to remove the JsonApiDotNetCore.Configuration.ISerializerOptions interface and update its usages to use Newtonsoft.Json.JsonSerializerSettings instead.