Implement sending ephemeral events to appservices (MSC2409)

*This issue was originally created by [**@bodqhrohro**](https://github.com/bodqhrohro) at .*

**Description:**

Dunno what's that for (probably some custom extension), but [matrix-appservice-node](https://github.com/matrix-org/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.