Add derived types to Suggestions

[Martijnvos]

Derived types declarations (similar to IRichMessage) are missing on BaseSuggestion, breaking serialization. This is shown with a simple test ran against the current master branch:

[TestClass]
public class SuggestionsTests
{
  [TestMethod]
  public void SerializationReplySuggestionTest()
  {
      var replySuggestion = new ReplySuggestion()
      {
          Label = "Some label",
          PostbackData = "LABEL",
          Description = "Description of the label",
          Media = new MediaContent("Test image", "https://example.com", "image/jpg")
      };
      var serialized= JsonSerializer.Serialize<SuggestionBase>(replySuggestion);
  }
}

serialized will then look as follows, missing the description field and media object:

{
  "action" : "reply",
  "label" : "Some label",
  "postbackdata" : "LABEL"
}

Couple extra notes:

• I've kept the casing and the discriminator itself consistent
• I've left in the Description field, though the summary says it only applies to Twitter which is now deprecated
• I've had to add a getter and setter to EndTime and StartTime in CalendarOptions for those fields to be picked up during serialization
• I'm considering this a patch version update since it's backwards compatible and fixes things that were previously overlooked