[enhancement]: Support VAPID for WebPush

[iNPUTmice]

Which feature or improvement would you like to request?

One of the great features of JMAP is the ability to receive WebPush notifications. However a good number of Push Services (Googles Firebase for example) require that the (JMAP) server authenticates itsself via VAPID (Voluntary Application Server Identification protocol)

I've written a draft RFC on how this can be done with JMAP https://datatracker.ietf.org/doc/draft-gultsch-jmap-webpush-vapid/ and implement client support in Ltt.rs.

There is a chance that there might be a few minor syntax changes on the JMAP side of things before the RFC is finalized but the heavy lifting is likely on the JMAP->push services side of things and that's not going to change.

Is your feature request related to a problem?

No response

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