Add CancellationToken parameter to GetAsyncEnumerator

src/System.Private.CoreLib/shared/System/Collections/Generic/IAsyncEnumerable.cs

@@ -2,14 +2,17 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Threading;
+
 namespace System.Collections.Generic
 {
     /// <summary>Exposes an enumerator that provides asynchronous iteration over values of a specified type.</summary>
     /// <typeparam name="T">The type of values to enumerate.</typeparam>
     public interface IAsyncEnumerable<out T>
     {
         /// <summary>Returns an enumerator that iterates asynchronously through the collection.</summary>
+        /// <param name="cancellationToken">A <see cref="CancellationToken"/> that may be used to cancel the asynchronous iteration.</param>
         /// <returns>An enumerator that can be used to iterate asynchronously through the collection.</returns>
-        IAsyncEnumerator<T> GetAsyncEnumerator();
+        IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken cancellationToken = default);
     }
 }