Merge typed role and rolegroup configs before converting to product-config maps

[nightkr]

Currently, overriding gets pretty tricky, since reasonable-looking code like this:

#[derive(Deserialize)]
struct FooConfig {
    #[serde(default = "FooConfig::default_port")]
    port: u16,
}

impl FooConfig {
    fn default_port() -> u16 {
        9191
    }
}

is subtly wrong, since the rolegroup's default port will override an explicitly specified role-level port. #282 made this somewhat worse, since it now triggers for all rolegroups (whereas previously it would only trigger for rolegroups that specified a config: field), but the issue was still there before.

[lfrancke]

@teozkr could you please add a link / quick explanation what this is blocked on?
Something in schemars if I'm correct

[lfrancke]

@teozkr ping

[nightkr]

Ah sorry, @lfrancke. This is blocked on GREsau/schemars#167, which is required in order to set up the correct type bounds in the generated schemas.