Allow notification handlers post-creation temporary registration (#223)

- We still allow notification handlers to be configured pre-creation, eliminating any race conditions that may result from missed notifications. But now we also allow for post-creation notification handler registration.
- Handlers may now also be removed. This permits temporary registrations that may be created in a scoped manner.
- Registration handling is thread-safe.
- Handlers are now cancelable.
- Exceptions from notification handlers trigger all normal exception handling in the dispatch pipeline.

This also:
- Adds a WithStreamServerTransport as a counterpart to WithStdioServerTransport. The latter was used in several tests in a hacky way, and that's now simplified via the former's existence.

src/ModelContextProtocol/Protocol/Types/ServerCapabilities.cs

@@ -45,5 +45,5 @@ public class ServerCapabilities
     /// The server will not re-enumerate the sequence.
     /// </remarks>
     [JsonIgnore]
-    public IEnumerable<KeyValuePair<string, Func<JsonRpcNotification, Task>>>? NotificationHandlers { get; set; }
+    public IEnumerable<KeyValuePair<string, Func<JsonRpcNotification, CancellationToken, Task>>>? NotificationHandlers { get; set; }
 }