Add trusted peers (#1640)

## Issue Addressed

Closes #1581 

## Proposed Changes

Adds a new cli option for trusted peers who always have the maximum possible score.

beacon_node/src/cli.rs

@@ -131,7 +131,13 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .help("Disables the discv5 discovery protocol. The node will not search for new peers or participate in the discovery protocol.")
                 .takes_value(false),
         )
-
+        .arg(
+            Arg::with_name("trusted-peers")
+                .long("trusted-peers")
+                .value_name("TRUSTED_PEERS")
+                .help("One or more comma-delimited trusted peer ids which always have the highest score according to the peer scoring system.")
+                .takes_value(true),
+        )
         /* REST API related arguments */
         .arg(
             Arg::with_name("http")

beacon_node/src/config.rs

@@ -2,7 +2,7 @@ use beacon_chain::builder::PUBKEY_CACHE_FILENAME;
 use clap::ArgMatches;
 use clap_utils::BAD_TESTNET_DIR_MESSAGE;
 use client::{config::DEFAULT_DATADIR, ClientConfig, ClientGenesis};
-use eth2_libp2p::{multiaddr::Protocol, Enr, Multiaddr, NetworkConfig};
+use eth2_libp2p::{multiaddr::Protocol, Enr, Multiaddr, NetworkConfig, PeerIdSerialized};
 use eth2_testnet_config::Eth2TestnetConfig;
 use slog::{crit, info, Logger};
 use ssz::Encode;
@@ -343,6 +343,17 @@ pub fn set_network_config(
             .collect::<Result<Vec<Multiaddr>, _>>()?;
     }
 
+    if let Some(trusted_peers_str) = cli_args.value_of("trusted-peers") {
+        config.trusted_peers = trusted_peers_str
+            .split(',')
+            .map(|peer_id| {
+                peer_id
+                    .parse()
+                    .map_err(|_| format!("Invalid trusted peer id: {}", peer_id))
+            })
+            .collect::<Result<Vec<PeerIdSerialized>, _>>()?;
+    }
+
     if let Some(enr_udp_port_str) = cli_args.value_of("enr-udp-port") {
         config.enr_udp_port = Some(
             enr_udp_port_str