Introduce BeginExecute to express fire-and-forget intent on ExecutionNode (#9432)

Author: michaelstaib

src/HotChocolate/Fusion/src/Fusion.Execution/Execution/ExecutionState.cs

@@ -133,7 +133,7 @@ public void StartNode(OperationPlanContext context, ExecutionNode node, Cancella
         }
 
         RemoveFromBacklog(node.Id, NodeStateNone);
-        _ = node.ExecuteAsync(context, cancellationToken);
+        node.BeginExecute(context, cancellationToken);
     }
 
     public void EnqueueForCompletion(ExecutionNodeResult result)

src/HotChocolate/Fusion/src/Fusion.Execution/Execution/Nodes/ExecutionNode.cs

@@ -54,7 +54,14 @@ public abstract class ExecutionNode : IOperationPlanNode, IEquatable<ExecutionNo
     /// </summary>
     public ReadOnlySpan<IOperationPlanNode> OptionalDependencies => _optionalDependencies;
 
-    public async Task ExecuteAsync(
+#pragma warning disable CA2012
+    public void BeginExecute(
+        OperationPlanContext context,
+        CancellationToken cancellationToken = default)
+        => _ = ExecuteAsync(context, cancellationToken);
+#pragma warning restore CA2012
+
+    private async ValueTask ExecuteAsync(
         OperationPlanContext context,
         CancellationToken cancellationToken = default)
     {