RootConfig.validate_config()
HomeServerConfig
This means we can move the open registration config validation from `setup()` to `HomeServerConfig.validate_config()` (much more sane). Spawning from looking at this area of code in https://github.com/element-hq/synapse/pull/19015
RootConfig
Config
MockClock()
Clock.call_when_running(...)
macaroon_secret_key_path