Refactor transports to help enable graceful shutdown (#142)

- This also paves the way for better multi-session support
- We should definitely rethink names for the transport API
- For now, I kept the names similar as possible, so we can
  focus on the API shape

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/ModelContextProtocol/Shared/NotificationHandlers.cs

@@ -0,0 +1,16 @@
+using ModelContextProtocol.Protocol.Messages;
+using System.Collections.Concurrent;
+
+namespace ModelContextProtocol.Shared;
+
+internal sealed class NotificationHandlers : ConcurrentDictionary<string, List<Func<JsonRpcNotification, Task>>>
+{
+    public void Add(string method, Func<JsonRpcNotification, Task> handler)
+    {
+        var handlers = GetOrAdd(method, _ => []);
+        lock (handlers)
+        {
+            handlers.Add(handler);
+        }
+    }
+}