Use UnityAPI on iOS when using Swift trampoline

[aurimasc]

https://jira.unity3d.com/browse/PLAT-21965
Change integration with the Swift trampoline to use Unity public API directly from Objective-C++ code without swift bridge.
The new implementation is much cleaner and safer, with a price that we lose compatibility with Unity 6.5, but that was accepted.

Tested on simulator using 6.6 and trunk that remote notifications have been correctly subscribed to and device token has been received.

[u-pr]

💡 Harness Review

The Swift-project notification observer integration now uses Unity’s public Swift API directly, and I examined the affected lifecycle path, package metadata, and the version-specific Xcode post-processing logic. The stated Unity 6.6 boundary is consistent with the direct header dependency; no actionable defects were substantiated.

_{Reviewed commit 8aa6e61}

_{🤖 Helpful? 👍/👎}