[clr-ios] Update iOS SDK build to use R2R for startup path

[kotlarmilos]

Description

Add MSBuild tasks and targets in the iOS SDK to control R2R, ensuring trimming, Crossgen2, native linking, and codesigning run in the correct order for Apple targets.

Tasks

• Add MSBuild properties/items for Apple targets (e.g., PublishReadyToRun, PublishTrimmed, ReadyToRunComposite, ObjFormat=macho)
• Extend targets to invoke Crossgen2 automatically with correct argumente
• Emit R2R as Mach-O objects and link into composite R2R dylibs placed in the app bundle
• Generate and preserve debug symbols (.dSYM) for R2R outputs
• Provide clear build logs showing effective properties and Crossgen2 command lines
• Validate dotnet publish -r ios-arm64 -c Release /p:PublishReadyToRun=true produces R2R outputs
• Combine R2R and interpreter in Debug builds

[rolfbjarne]

Completed in dotnet/macios#24437.