[swift4] Add swift package manager and update dependencies (#4141)

* [swift4] add Swift Package Manager

* [swift] update petstore projects

* [swift4] try to run CI again

samples/client/test/swift4/default/Package.swift

@@ -0,0 +1,27 @@
+// swift-tools-version:4.2
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "TestClient",
+    products: [
+        // Products define the executables and libraries produced by a package, and make them visible to other packages.
+        .library(
+            name: "TestClient",
+            targets: ["TestClient"])
+    ],
+    dependencies: [
+        // Dependencies declare other packages that this package depends on.
+        .package(url: "https://github.com/Alamofire/Alamofire.git", from: "4.9.0")
+    ],
+    targets: [
+        // Targets are the basic building blocks of a package. A target can define a module or a test suite.
+        // Targets can depend on other targets in this package, and on products in packages which this package depends on.
+        .target(
+            name: "TestClient",
+            dependencies: ["Alamofire"],
+            path: "TestClient/Classes"
+        )
+    ]
+)