WebAPI: Support push notification with WebPush

[tehcneko]

Implemented RFC 8291 and RFC 8292.
You can read more info about Web Push API at MDN:
https://developer.mozilla.org/en-US/docs/Web/API/Push_API
Web Push functionalities in WebUI are implemented in pull request #23096.

---

API Changes:

• Add push/subscribe endpoint with parameter subscription for registering WebPush notifications
  subscription can be obtained from PushManager.subscribe() in browser, eg:

{
    "endpoint": "https://fcm.googleapis.com/fcm/send/[REDACTED]", 
    "keys": {
        "p256dh": "[REDACTED]", 
        "auth": "[REDACTED]"
    }
}

• Add push/subscriptions endpoint for listing all subscribed WebPush subscriptions
• Add push/test endpoint for testing WebPush notifications
• Add push/unsubscribe endpoint with parameter endpoint for unregistering WebPush notifications
• Add push/vapidPublicKey endpoint for retrieving the VAPID public key for WebPush notifications subscribing

---

Push payloads are in following format:

{
    "event": "torrent_finished",
    "payload": {
        "torrent_name": "Test",
     }
}

Currently supported events: test, torrent_added, torrent_finished, full_disk_error, add_torrent_failed.

---

Closes #16414

[glassez]

@tehcneko
Could you describe for those who are out of topic, how it is supposed to be used? Preferably in enough detail that someone can test it.

[tehcneko]

@glassez You can test it with WebUI from #23096.

[github-actions]

This PR is stale because it has been 60 days with no activity. This PR will be automatically closed within 7 days if there is no further activity.

[tehcneko]

Yeah, it has been a while...