[.NET] Optimize support for app extensions

[rolfbjarne]

We currently treat app extensions like separate apps, which means there are a lot of duplication.

This will be fixed by:

• Deduplication of runtime native libraries (libmono, libxamarin). This requires having these as frameworks, putting them in the main app only and having a rpath=../../Frameworks in the extension's native executable.
  • Blocked for libmono because we don't have libmono as a framework yet (Mono: the runtime packages for .NET 6+ do not contain the runtime as a .framework for iOS/tvOS/watchOS dotnet/runtime#42846). We can still do this for libxamarin though.
• Deduplication of AOT-ed assemblies. This requires building assemblies to frameworks.
• Deduplication of linked output. This requires running the linker only once for all extensions + the main app, with all the assemblies in one big closure.
• Deduplicate user frameworks between the app extensions and the app (tracked here: Put user frameworks in the container app bundle for app extensions #17876).
• Deduplicate icudt.dat between app extensions and the app (tracked here: Use the app container's icudt.dat file in app extensions #17877)

[rolfbjarne]

This will not be done in the .NET 6 time frame due to time constraints, so moving to .NET 7.

[rolfbjarne]

This will not be done in the .NET 7 time frame due to time constraints, so moving to .NET 8.

[SaqibS]

How likely is this to be done in the .NET 8 timeframe? We're actively blocked by this, but want to move off of Xamarin.iOS due to end of support coming up. Appreciate any visibility into timelines.

[rolfbjarne]

@SaqibS it's unlikely that all of the tasks here will be completed, but we plan on implementing some of them. That said, this also depends on what kinds of surprises Apple has in store for us at WWDC and later over the summer.