Add support for iOS

[Angelina-C]

Currently, only Android is supported. Appium should also work on iOS. One culprit is that it requires iTunes and Xcode, of which the latter is only available on macOS. A first step is to use a Mac for running Appium on iOS devices. https://medium.com/@abhaykhs/using-appium-to-run-ios-tests-on-real-devices-fabd9850a06a

The next step is to investigate if it is possible on Linux as well. See this article for running XCode on Linux. https://bartsaintgermain.medium.com/ios-automation-on-linux-using-appium-f35bc14e86eb

Note that the iOS apps are probably different from the Android apps. The app-specific Appium scripts thus need to be written again for iOS. Do try to keep the function names the same across the platforms, so send_message in WhatsApp Android does the same as send_message in WhatsApp iOS.