Make NetworkActor generic over `NetworkDriver` and `NetworkHandler`

[op-will]

NetworkActor is tightly coupled to NetworkDriver/NetworkHandler. The goal of this task is to decouple them. There may be a better way, but this probably entails:

1. Codifying the interface between NetworkActor and NetworkHandler as a trait
2. Codifying the interface between NetworkActor and NetworkDriver as a trait (this trait will be generic over the new NetworkHandler trait)
3. Making NetworkActor generic over both
4. Injecting the NetworkDriver trait into NetworkActor

It is also very possible that some or all of NetworkHandler should be consolidated into NetworkActor. Good candidates for consolidation are the following, but it is possible that gossip and discovery also belong:

    /// The receiver for the ENRs.
    pub enr_receiver: mpsc::Receiver<Enr>,
    /// The sender for the unsafe block signer.
    pub unsafe_block_signer_sender: watch::Sender<Address>,
    /// The peer score inspector. Is used to ban peers that are below a given threshold.
    pub peer_score_inspector: tokio::time::Interval,
    /// A handler for the block signer.
    pub signer: Option<BlockSignerHandler>,