iOS: requestPushAuthorization permission to read

[sashkaizen]

Overview

If the application had previously granted push-notification permissions prior to utilziing exponea then exponea wont read from those permissions.

• Updated iOS push authorization flow to read existing notification permission state first using UNUserNotificationCenter.getNotificationSettings.
• Kept original method semantics (Future<bool> / FlutterResult boolean):
• Returns true and registers for remote notifications when status is already .authorized or .provisional.
• Requests permission only when status is .notDetermined.
• Returns false when status is .denied.
• Ensured UIApplication.shared.registerForRemoteNotifications() is called only in “granted” paths (same practical contract as before).
• Added handling for .ephemeral and @unknown default to make behavior explicit and forward-safe.
• Result: fixes upgrade scenario where previously granted permissions were not reliably reflected by the SDK flow, without changing the public API contract.