This allows us to get access to `server_name` so we can use it when
creating the `LoggingContext("main")` in the future (pre-requisite for
https://github.com/element-hq/synapse/pull/18868).
This also allows us more flexibility to parse config however we want and
setup a Synapse homeserver. Like what we do in [Synapse Pro for Small
Hosts](https://github.com/element-hq/synapse-small-hosts).
Split out from https://github.com/element-hq/synapse/pull/18868