[WIP] Ability to create and display xamarin forms page in an ios extension

[alexeystrakh]

This PR shows how to use Xamarin.Forms pages in an iOS extensions. This will allow to move all the native iOS controllers to the shared XF core and implement them with Xaml, MVVM and hot reload.

The key steps are:

1. Add XF NuGet reference
2. initialize XF
3. Do NOT load XF app
4. Create XF pages and convert them to view controller with CreateViewController method
5. Stay alert on memory usage by your extension as XF will add memory pressure and extensions are strict on that.

I've tested on sim and device, in release configuration with enabled linker (SDK Only or All) the extension performs well, no issues found with this approach.

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[eliykat]

Hi @alexeystrakh, thanks for the submission! If you're still interested in progressing this PR, can you please sign the CLA (see instructions above). I'll then schedule it for further review.

[mpbw2]

@eliykat We can actually close this one - Alexey is from the Xamarin team and created this for demonstration purposes.