Put ListenConfig inside Discv5Config

[ackintosh]

ref: sigp#160 (comment)

Before

// builder
let config = Discv5ConfigBuilder::default().build();
// default configuration
// let config = Dicv5Config::default();

let listen_config = ListenConfig { ... (omitted)... }

let mut discv5: Discv5 = Discv5::new(enr, enr_key, config, listen_config).unwrap();

After

• Discv5ConfigBuilder::new() requires listen_config since ListenConfig is not optional.
• Discv5 and Discv5ConfigBuilder no longer implement Default since we can not determine the default value for ListenConfig.

let listen_config = ListenConfig { ... (omitted)... }
let config = Discv5ConfigBuilder::new(listen_config).build();

let mut discv5: Discv5 = Discv5::new(enr, enr_key, config).unwrap();

[ackintosh]

@AgeManning @divagant-martian This PR is ready for review. 🙏