Support for standalone iOS native framework

This revision allows the installation of the Kaleyra Video iOS native SDK as a standalone framework (without WebRTC framework).
You should use the standalone framework only in case there's a conflict with another module providing the WebRTC framework.

Caution

Beware, the WebRTC framework is still needed in order to compile and run the application when using the Kaleyra Video SDK. Moreover, you are responsible for using a compatible WebRTC module. WebRTC versions greater than or equal to M100 should work fine.