Allow load server config from file and env vars

[gnieto]

Use mehcode/config crate to configure the sources and priorites for configuration loader.

I've moved all structs on server/mod.rs to a config module and I've splitted in two distinct structures:

• RawConfig (I couldn't find a better name) which holds the configuration that can be changed by the users (via a TOML file or env vars)
• Config: The calculated and parsed version, which holds all the materialized configuration.

As CONFIG is a static value and is used as a singleton on some places, I've needed to add the MAELSTROM_CONFIG_FILE env var to allow loading from a distinct file on tests.

I wanted to ask some questions:

• Are any concerns on depending on mehcode/config and/or anyhow?
• Which config format is prefered? TOML? YAML? dotenv?
• Ia there any problem with prefix all env vars with MAELSTORM?

[chrisabruce]

No concerns with that Config crate or Anyhow as of now. We should use yaml. That is the default for other Home Server admins, so best to be consistent. As for prefix, probably ok. Maybe worth looking at what Synapse and Dendrite do, but we can always adjust later.

[gnieto]

Changed to yaml