refactor(server/config): Implement checked access to server configuration, decoupled from SessionManager

[whisperity]

⚠️ Full description WIP!

This commit introduces a new ServerConfiguration handler object which
duty is to deal with all the handling of the configuration options the
servers are started with. Previously, this was done by SessionManager
since a5119f0 (#1249) introduced
non-authentication-related configuration options into the previously
authentication-only configuration file.

The new infrastructure aims to offer a generally user-friendly and more
type-safe way of dealing with configuration options in various parts of
the server package. Options are registered simply akin to adding
options to an argparse parser.

Moved the logic of finding or creating an example server config to the
new class.

The first option, worker_processes has already been refactored into
being handled by the new instance, but there are more to follow...