Add ability for configure Validation property throught PaymentProviderSettingAttribute

[PeterKvayt]

Hi!

I did refactoring for Wordlpay payment provider and faced with following problem.

I want to make some settings mandatory. It enhance user expirience, when users won't be able to save payment method if not all mandatory fields were provided.

I see it in following way: we have PaymentProviderSettingAttribute and UmbracoCommerceSettingAttribute:

[AttributeUsage(AttributeTargets.Property)]
public class PaymentProviderSettingAttribute : UmbracoCommerceSettingAttribute
{
    public bool IsAdvanced { get; set; }
}

[AttributeUsage(AttributeTargets.Property)]
public class UmbracoCommerceSettingAttribute : Attribute
{
    public string Key { get; set; }

    public string Name { get; set; }

    public string Description { get; set; }

    public string View { get; set; }

    public string Config { get; set; }

    public int SortOrder { get; set; }
}

I think it would be great if we add Validation as it was done with Config property. And with the help of property Validation we will be able to manage mandatory state.