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/lib.rs

@@ -26,4 +26,4 @@ pub use metrics::scrape_discovery_metrics;
 pub use peer_manager::{
     client::Client, score::PeerAction, PeerDB, PeerInfo, PeerSyncStatus, SyncInfo,
 };
-pub use service::{Libp2pEvent, Service, NETWORK_KEY_FILENAME};
+pub use service::{load_private_key, Libp2pEvent, Service, NETWORK_KEY_FILENAME};