Remove `#[derive(clap::Parser)]` from `Config`

[davidpdrsn]

When integrating this into our internal services we ran into a few oddities that were caused by having #[derive(clap::Parser)] on Config:

• Docs on the fields becomes help text for CLI arguments. You cannot specify the two separately which means things like default values are repeated in the help text, once from the doc comment and once #[clap(default_value = "...")].
• Not clear whether Config's Default implementation should call clap or not. Getting env vars in Default is kinda odd but if you don't you have to duplicate the default values in the code.
• Users might wanna customize the prefix used to env vars, that wasn't previously possible.

With this change Config is becomes just a regular struct. Users can then build then own CLI arg parsing and convert between their config and our config however they want. Thats a bit less convenient but probably worth it overall.

[repi]

if one end up with a lot of small service apps using this and wanting to reduce boilerplate with them one could add this as an optional clap feature that has a small conversion struct from this config to a clap-compatible config. for a fully batteries-included (but optional) approach.

but starting without it and with this feels like the easiest and cleanest.