Stabilize support for sending ephemeral events to application services, as per MSC2409

[jason-famedly]

Part of #18118

MSC2409 was merged into the spec on Oct 28, 2024 and is part of Matrix Spec 1.13

This adds the stable identifiers for MSC2409 while maintaining the existing unstable identifier for backwards compatibility. The existing to_device identifiers borrowed from MSC2409 for MSC4203 are also maintained.

(Includes one drive-by removal of a FIXME comment that was no longer needed after May 16, 2022)

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[sandhose]

I'm not sure how we handled that in the past, but I feel like we should continue sending the unstable identifier to avoid breaking application services which only support the unstable prefix?

[jason-famedly]

Hmmm. That is fair. Do you think using the change in the configuration setting should be the decider on which identifier should be sent? So that if the config says receive_ephemeral it would provide the stable key, but if it was de.sorunome.msc2409.push_ephemeral it would use the older unstable key? Or do you suppose just sending both each time?

[jason-famedly]

Gave the first option a try in 5c76d3f.

As a note: the existing code did not do the optional exclusion of the key if there was no relevant data. Would you be interested in that here, or push it to later?