feat: add ref helpers for act/reaction (#91)

Author: ferhatelmas

src/Models/Activity.cs

@@ -11,6 +11,8 @@ public abstract class ActivityBase : CustomDataBase
         public DateTime? Time { get; set; }
         public List<string> To { get; set; }
         public float? Score { get; set; }
+
+        public string Ref() => $"SA:{Id}";
     }
 
     public class Activity : ActivityBase

src/Models/Reaction.cs

@@ -26,6 +26,8 @@ public class Reaction
 
         [JsonProperty(NullValueHandling = NullValueHandling.Ignore, PropertyName = "children_counts")]
         public Dictionary<string, int> ChildrenCounters { get; set; }
+
+        public string Ref() => $"SR:{Id}";
     }
 
     public class ReactionsWithActivity : GenericGetResponse<Reaction>