Add Rate-Topics header to WebSocket connection and JSON stream

[wunter8]

fixes binwiederhier/ntfy#652

[binwiederhier]

So here are my unfiltered thoughts: Would "rate topics" be useful outside of unifiedpush maybe in the future? Like, would it be useful to say "this is a rate topic, I will be take all the rate limits for the messages that come in" to a user?

I am not saying that we should implement this feature, just maybe that in the code we call them "rateTopics"? Thoughts?

[wunter8]

Since it's correct for the current use case, I'd say don't change it now because we might want something different later. I think if we implement the new feature in the future, we can change the terminology then? 🤷‍♂️

[binwiederhier]

Fair enough