Port xtro to use .NET instead of Mono

[rolfbjarne]

FWI I believe xtro's usage of objective-sharpie/Mono is the only reason we still need Mono during our build (ref: #21411).

This can probably only be done once Objective-Sharpie has been ported to .NET.

[dalexsoto]

@rolfbjarne I could be wrong but wasn’t Sharpie self-contained? (About it using system mono)

[rolfbjarne]

@rolfbjarne I could be wrong but wasn’t Sharpie self-contained? (About it using system mono)

I'm not sure I understand - Objective-Sharpie is self-contained, but our xtro tests consumes dylibs from Objective-Sharpie:

https://github.com/xamarin/xamarin-macios/blob/25fa00fbca0e763167d6fd99e3401637d8500647/tests/xtro-sharpie/xtro-sharpie/xtro-sharpie.csproj#L41

and that won't work in .NET.

[dalexsoto]

Ah right 🤦‍♂️ (needs more coffee)