Update SubscriptionState

Include missing 'awaiting_signup' subscription state.

(See https://maxio-chargify.zendesk.com/hc/en-us/articles/5404222005773-Subscription-States#subscription-states-0-0).

Author: pro-pete

Source/ChargifyDotNet/Interfaces/ISubscription.cs

@@ -127,6 +127,10 @@ public enum SubscriptionState
         /// </summary>
         Paused,
         /// <summary>
+        /// The 'awaiting_signup' subscription state
+        /// </summary>
+        Awaiting_Signup,
+        /// <summary>
         /// The 'unknown' subscription state, only internal to this wrapper
         /// </summary>
         Unknown = -1