Allow adjusting deserialisation of responses

[Smaug123]

Currently we just use readJson, but RestEase itself can control this with a runtime JsonSerializer:

var settings = new JsonSerializerSettings()
{
    ContractResolver = new CamelCasePropertyNamesContractResolver(),
    Converters = { new StringEnumConverter() }
};

var api = new RestClient("https://api.example.com")
{
    JsonSerializerSettings = settings
}.For<ISomeApi>();