Add push flags

[abnegate]

What does this PR do?

Add new push flags:

• contentAvailable (iOS only): when set, notifications will wake up the app in the background to deliver the notification
• critical (iOS only): when set, tries to mark notification as "critical" and ignore do not disturb settings. Requires app to be granted the critical notification entitlement.
• priority. Let's you set either normal or high priority for notifications. Normal priority will deliver at the most convenient time depending on multiple factors

Test Plan

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Related PRs and Issues

• Add new push message parameters appwrite/appwrite#9060
• Support for Critical Alerts in Messaging appwrite/appwrite#8977

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