Support native plaform Http handlers for Xamarin.Mac

[tipa]

This PR adds support for native http handler when targeting Xamarin.Mac (NSUrlSessionHandler)

Fixes

#413 Cannot download multiple OneDrive files concurrently since v.1.13.0-beta

[msftclas]

All CLA requirements met.

[tipa]

Any timeline when the msgraph-sdk-dontnet package will use v1.21.0 which contains this change so I can stop using a workaround in my code?

[MIchaelMainer]

@tipa Thank you for following up. Can you confirm that this works as expected in this preview: https://www.nuget.org/packages/Microsoft.Graph.Core/1.21.0-preview.1

[tipa]

Unfortunately I cannot test it at the moment as VS for Mac doesn't start on my updated Big Sur machine (https://developercommunity.visualstudio.com/content/problem/1088817/visual-studio-2019-864-crashes-on-mac-os-big-sur.html).
Is it possible the change was merged into 1.20.1 already? That's the package that I am currently using and it seems to work as expected.

[MIchaelMainer]

No, it isn't in 1.20.1. You have to be using your workaround for that.