Implement sending ephemeral events to appservices (MSC2409)

[bodqhrohro]

Description:

Dunno what's that for (probably some custom extension), but matrix-appservice-node generates configs with that key, and Dendrite fails to read it, so I had to remove the following line manually. Ideally, the config parser should just ignore unknown keys.

[tulir]

This should probably be split into two issues:

1. Ignore unknown keys in appservice registration files
2. Implement sending ephemeral events to appservices (MSC2409)

[bodqhrohro]

Done, I've split the parsing issue out: #1567

[Kab1r]

It's been a while and ignoring unknown keys is solved, so I might take a look at implementing this

[Blintmester]

Is there any progress on this? I am excited to see when it will be ready!

[sewnie]

Anything new?