feat(iOS): add support for iOS Live Activity push-to-start payloads

[johnsharp97]

Live Activities can be started remotely via push as of iOS 17.2. This adds the fields required to send a push-to-start notification through gorush.

• Add iOS Live Activity push-to-start support via attributes-type, attributes, and relevance-score
• Update README

Tested end to end on a physical device.

Reference: https://developer.apple.com/documentation/activitykit/starting-and-updating-live-activities-with-activitykit-push-notifications

[Copilot]

Pull request overview

Adds support for iOS Live Activity push-to-start payload fields so gorush can construct the APNs aps keys required to start Live Activities remotely (iOS 17.2+), alongside documentation and test updates.

Changes:

• Extend the iOS PushNotification model with attributes-type, attributes, and relevance-score.
• Populate these fields in the APNs payload builder for Live Activity notifications.
• Update README and add/extend APNs payload unit tests for Live Activity scenarios (including push-to-start).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
README.md	Documents liveactivity push type and new Live Activity push-to-start fields + example payload
notify/notification.go	Adds Live Activity push-to-start fields to the request model
notify/notification_apns.go	Maps new request fields into the APNs aps payload for Live Activities
notify/notification_apns_test.go	Adds assertions and a new test covering push-to-start payload composition

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[appleboy]

@johnsharp97 Thanks.