Add support for iPhone and iPad debugging over USB

[citizenmatt]

Rider can already debug iOS devices over wifi. This PR adds support for debugging via USB.

• Loads Unity's native extension to enumerate devices, and lists all connected devices in the "Attach to Unity Process" dialog. The list is refreshed while the dialog is open. The native extension does not give much information - it simply provides device and product ID. This can be used to look up a device type (iPad/iPhone), but there are no other details such as model number (e.g. iPhone 7) or device name (Matt's iPhone 7). There is also no indication that there is a Unity application running on the device.
• The native extension doesn't unload cleanly, so must be used out of process - a helper .exe has been added to do this. The helper .exe simply loads the native library and starts polling for devices. The "Attach to Unity Process" dialog starts the process when the dialog is first shown, listens to its output, and gracefully shuts it down when the dialog is closed.
• When the debugger is launched, it will load the native extension and set up a proxy from a local socket (hard coded as 12000, as per Unity's own debugger plugins) to port 56000 on the device (again hard coded)

This PR does not address USB debugging for Android.