Associate the "optional" boolean flag with each app settings file

[Arash-Sabet]

The provided app settings files may be optional and depending on the scenarios but the current code does not support it. We need to make the following enhancements:

• The current implementation of GetConfigurationFiles() method to be marked as [Obsolete] to generate a proper message to direct developers to the new method
• Introduce a new abstract method called GetTestAppSettings() to return a collection of the following class instead of strings:

public class TestAppSettings
{
   public string? Filename { get; set; }
   public bool IsOptional { get; set; } = false;
}