reuse beacon_node methods for initializing network configs in boot_node (#1520)

## Issue Addressed

#1378

## Proposed Changes

Boot node reuses code from beacon_node to initialize network config. This also enables using the network directory to store/load the enr and the private key.

## Additional Info

Note that before this PR the port cli arguments were off (the argument was named `enr-port` but used as `boot-node-enr-port`).
Therefore as port always the cli port argument was used (for both enr and listening). Now the enr-port argument can be used to overwrite the listening port as the public port others should connect to.

Last but not least note, that this restructuring reuses `ethlibp2p::NetworkConfig` that has many more options than the ones used in the boot node. For example the network config has an own `discv5_config` field that gets never used in the boot node and instead another `Discv5Config` gets created later in the boot node process.

Co-authored-by: Age Manning <Age@AgeManning.com>

beacon_node/eth2_libp2p/src/service.rs

@@ -357,7 +357,7 @@ fn keypair_from_bytes(mut bytes: Vec<u8>) -> error::Result<Keypair> {
 /// generated and is then saved to disk.
 ///
 /// Currently only secp256k1 keys are allowed, as these are the only keys supported by discv5.
-fn load_private_key(config: &NetworkConfig, log: &slog::Logger) -> Keypair {
+pub fn load_private_key(config: &NetworkConfig, log: &slog::Logger) -> Keypair {
     // check for key from disk
     let network_key_f = config.network_dir.join(NETWORK_KEY_FILENAME);
     if let Ok(mut network_key_file) = File::open(network_key_f.clone()) {