config: add settings for status service

[lucab]

This applies to both graph-builder and policy-engine.

The status service needs to configurable via TOML config file. Initial proposal:

#[derive(Deserialize)]
struct FileConfig {
    /// Status service config.
    status: Option<StatusConfig>,
}

#[derive(Deserialize)]
struct StatusConfig {
    /// Address on which the status service will listen,
    pub address: Option<IpAddr>,

    /// Port to which the status service will bind.
    pub port: Option<u16>,
}