L2, L1: Refactor ethrex.rs's logic to have a launcher for L1 and another for L2

[ilitteri]

To have a cleaner code it'd be good to separate the L1 and L2 launch logic even if we have to duplicate some code. Having ethrex.rs's main functions as follows would be very nice:

#[tokio::main]
async fn main() {
    let matches = cli::cli().get_matches();
    cfg_if::cfg_if! {
        if #[cfg(feature = "l2")] {
            launch_l2(...)
        } else {
            launch_l1(...)
        }
    }
}

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