[Swift5] Add an async execute API to RequestBuilder<T> (#14416)

* Add async `execute` interface

* Update samples

Run `./bin/generate-samples.sh`

* Add an explicit `self`

I forgot to add it

* Add an availability condition

modules/openapi-generator/src/main/resources/swift5/APIs.mustache

@@ -65,6 +65,33 @@ import Vapor
         return requestTask
     }
 
+    {{#useAsyncAwait}}
+    @available(macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
+    @discardableResult
+    {{#nonPublicApi}}internal{{/nonPublicApi}}{{^nonPublicApi}}open{{/nonPublicApi}} func execute() async throws -> Response<T> {
+        return try await withTaskCancellationHandler {
+            try Task.checkCancellation()
+            return try await withCheckedThrowingContinuation { continuation in
+                guard !Task.isCancelled else {
+                  continuation.resume(throwing: CancellationError())
+                  return
+                }
+
+                self.execute { result in
+                    switch result {
+                    case let .success(response):
+                        continuation.resume(returning: response)
+                    case let .failure(error):
+                        continuation.resume(throwing: error)
+                    }
+                }
+            }
+        } onCancel: {
+            self.requestTask.cancel()
+        }
+    }
+    
+    {{/useAsyncAwait}}
     {{#nonPublicApi}}internal{{/nonPublicApi}}{{^nonPublicApi}}public{{/nonPublicApi}} func addHeader(name: String, value: String) -> Self {
         if !value.isEmpty {
             headers[name] = value

samples/client/petstore/swift5/asyncAwaitLibrary/PetstoreClient/Classes/OpenAPIs/APIs.swift

@@ -56,6 +56,31 @@ open class RequestBuilder<T> {
         return requestTask
     }
 
+    @available(macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
+    @discardableResult
+    open func execute() async throws -> Response<T> {
+        return try await withTaskCancellationHandler {
+            try Task.checkCancellation()
+            return try await withCheckedThrowingContinuation { continuation in
+                guard !Task.isCancelled else {
+                  continuation.resume(throwing: CancellationError())
+                  return
+                }
+
+                self.execute { result in
+                    switch result {
+                    case let .success(response):
+                        continuation.resume(returning: response)
+                    case let .failure(error):
+                        continuation.resume(throwing: error)
+                    }
+                }
+            }
+        } onCancel: {
+            self.requestTask.cancel()
+        }
+    }
+    
     public func addHeader(name: String, value: String) -> Self {
         if !value.isEmpty {
             headers[name] = value