chore: os version in useragent for foundation based OS (#394)

Author: wooj2

Packages/ClientRuntime/Sources/Util/PlatformOperationSystemVersion.swift

@@ -0,0 +1,24 @@
+//
+// Copyright Amazon.com Inc. or its affiliates.
+// All Rights Reserved.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+#if os(iOS) || os (watchOS) || os(macOS) || os(tvOS)
+import Foundation.NSProcessInfo
+
+public struct PlatformOperationSystemVersion {
+    static public func operatingSystemVersion() -> String? {
+        let osVersion = ProcessInfo.processInfo.operatingSystemVersion
+        return "\(osVersion.majorVersion).\(osVersion.minorVersion).\(osVersion.patchVersion)"
+    }
+}
+#else
+// TODO: Implement for Linux & Windows
+public struct PlatformOperationSystemVersion {
+    static public func operatingSystemVersion() -> String? {
+        return nil
+    }
+}
+#endif