Execute load test for webpush

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<h3>Goal:</h3>
<ul>
<li>To observe how Coder handles web push notification usage at scale.</li>
</ul>

User target | coderd replicas | Subscribed users | Notification frequency
-- | -- | -- | --
1k | 3 | 200 | 6/minute
2k | 3 | 400 | 6/minute
3k | 4 | 600 | 6/minute
10k | 10 | 2000 | 6/minute


c.f. #1160 for load generator