Added & modified several message types.

Requests/Callbacks/CallbackManager.cs

@@ -0,0 +1,43 @@
+namespace HermesSocketLibrary.Requests.Callbacks
+{
+    public class CallbackManager<A> : ICallbackManager<A> where A : class
+    {
+        private readonly IDictionary<string, A> _callbacks;
+
+
+        public CallbackManager()
+        {
+            _callbacks = new Dictionary<string, A>();
+        }
+
+
+        public string GenerateKeyForCallback(A callback)
+        {
+            string key = GenerateKey();
+            _callbacks.Add(key, callback);
+            return key;
+        }
+
+        public A? Get(string key)
+        {
+            if (_callbacks.TryGetValue(key, out var callback))
+                return callback;
+            return null;
+        }
+
+        public A? Take(string key)
+        {
+            if (_callbacks.TryGetValue(key, out var callback))
+            {
+                _callbacks.Remove(key);
+                return callback;
+            }
+            return null;
+        }
+
+        private string GenerateKey()
+        {
+            return Guid.NewGuid().ToString("D");
+        }
+    }
+}

Requests/Callbacks/ICallbackManager.cs

@@ -0,0 +1,9 @@
+namespace HermesSocketLibrary.Requests.Callbacks
+{
+    public interface ICallbackManager<A> where A : class
+    {
+        string GenerateKeyForCallback(A callback);
+        A? Get(string key);
+        A? Take(string key);
+    }
+}