Interruption Level for APS to deliver time-sensitive notifications for IOS

[cyberpwnn]

fixes #80

Simply adds time-sensitive as a property for APS and allows it to play instead of getting rejected on firebases end.

You can now deliver time sensitive notifications with sound on IOS

• Added String? interruptionLevel and String? soundRaw to the Aps model
• If sound is defined, that is serialized, otherwise soundRaw is, if neither are defined, the sound field is not included in the map.

MulticastMessage(
  tokens: ...,
  data: ...,
  notification: Notification(title: title, body: body),
  apns: ApnsConfig(
    headers: {"apns-priority": "5"}, // Firebase requires 5, anything higher is rejected
    payload: ApnsPayload(
      aps: Aps(
        soundRaw: "default", // Makes it play a sound on ios. Using "CriticalSound" object rejects the notification on firebase.
        interruptionLevel: "time-sensitive", // Actually makes the notification time sensitive
      ),
    ),
  ),
)

This is the correct way to deliver Aps payload with time-sensitive and with actual sound, the CriticalSound object is not up to aps spec and it just gets rejected by firebase, notifications wont even deliver using that.

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