PropertyNamingPolicy null walue

[amoraller]

I think, we should support unsetted value for JsonRpcServerOptions.DefaultDataJsonSerializerOptions.PropertyNamingPolicy

Today, i use this policy

class UnsetNamingPolicy : JsonNamingPolicy
{
    public override string ConvertName(string name) => name;
}

For example System.Text.Json support null value for policy https://github.com/dotnet/corefx/blob/e667c29636a622eb4f9493f75232b44e0ae90b29/src/System.Text.Json/src/System/Text/Json/Serialization/JsonPropertyInfo.cs#L106-L119