Reference event service implementation (#811)

* Reference event service implementation

* Fix IReferenceable implementation of Id

* add structure to event body

src/Core/Enums/ReferenceEventSource.cs

@@ -0,0 +1,12 @@
+using System.Runtime.Serialization;
+
+namespace Bit.Core.Enums
+{
+    public enum ReferenceEventSource
+    {
+        [EnumMember(Value = "organization")]
+        Organization,
+        [EnumMember(Value = "user")]
+        User,
+    }
+}