Migrate Unity iOS/macOS native integration

[melekr]

Changes summary

This PR migrates the Unity native integration from the legacy cocoa static libraries to dynamic backtrace-cocoa frameworks:

• iOS: link Backtrace.xcframework (dynamic) + CrashReporter.xcframework (static), embed Backtrace, set Swift/platform flags via a post-build.
• macOS: Update plugin bundle (BacktraceMacUnity.bundle) that exports the native C API and nests Backtrace.framework for runtime. PLCrashReporter is linked statically.

The public C# API remains unchanged, only NativeClient and build steps are updated.

Why this change

• Modern packaging: XCFramework is Apple’s native distribution format for binary SDKs. Bundles device & simulator slices + correct slice selection at build time and Privacy & resources packaging.
• Stability/maintainability: Native standard Apple linking/embedding semantics + Swift runtime embedding of Swift stdlibs.
• Parity: aligns Unity integration with Backtrace’s official iOS/macOS SDK expectations.

Footprint

Sample Reports

What's next

• Expose more cocoa settings & expand configuration
• Offload/remove frameworks and bundle from repo