Add support for transformer-based WithOpenApi implementation

Author: captainsafia

src/OpenApi/src/Extensions/OpenApiEndpointConventionBuilderExtensions.cs

@@ -58,6 +58,12 @@ public static TBuilder WithOpenApi<TBuilder>(this TBuilder builder, Func<OpenApi
         return builder;
     }
 
+    public static TBuilder WithOpenApiTransformer<TBuilder>(this TBuilder builder, Func<OpenApiOperation, OpenApiOperationTransformerContext, CancellationToken, Task> transformer) where TBuilder : IEndpointConventionBuilder
+    {
+        builder.WithMetadata(new DelegateOpenApiOperationTransformer(transformer));
+        return builder;
+    }
+
     [RequiresDynamicCode(TrimWarningMessage)]
     [RequiresUnreferencedCode(TrimWarningMessage)]
     private static void AddAndConfigureOperationForEndpoint(EndpointBuilder endpointBuilder, Func<OpenApiOperation, OpenApiOperation>? configure = null)

src/OpenApi/src/PublicAPI.Unshipped.txt

@@ -1 +1,2 @@
 #nullable enable
+static Microsoft.AspNetCore.Builder.OpenApiEndpointConventionBuilderExtensions.WithOperationTransformer<TBuilder>(this TBuilder builder, System.Func<Microsoft.OpenApi.Models.OpenApiOperation!, Microsoft.AspNetCore.OpenApi.OpenApiOperationTransformerContext!, System.Threading.CancellationToken, System.Threading.Tasks.Task!>! transformer) -> TBuilder

src/OpenApi/src/Services/OpenApiDocumentService.cs

@@ -262,6 +262,14 @@ private async Task<Dictionary<OperationType, OpenApiOperation>> GetOperationsAsy
                 var transformer = operationTransformers[i];
                 await transformer.TransformAsync(operation, operationContext, cancellationToken);
             }
+
+            var endpointOperationTransformer = description.ActionDescriptor.EndpointMetadata
+                .OfType<DelegateOpenApiOperationTransformer>()
+                .LastOrDefault();
+            if (endpointOperationTransformer is not null)
+            {
+                await endpointOperationTransformer.TransformAsync(operation, operationContext, cancellationToken);
+            }
         }
         return operations;
     }