add checkpoint-sync-url-timeout flag (#3710)

## Issue Addressed
#3702 
Which issue # does this PR address?
#3702
## Proposed Changes
Added checkpoint-sync-url-timeout flag to cli. Added timeout field to ClientGenesis::CheckpointSyncUrl to utilize timeout set

## Additional Info

Please provide any additional information. For example, future considerations
or information useful for reviewers.


Co-authored-by: GeemoCandama <104614073+GeemoCandama@users.noreply.github.com>
Co-authored-by: Michael Sproul <micsproul@gmail.com>

beacon_node/src/cli.rs

@@ -714,6 +714,14 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .takes_value(true)
                 .conflicts_with("checkpoint-state")
         )
+        .arg(
+            Arg::with_name("checkpoint-sync-url-timeout")
+                .long("checkpoint-sync-url-timeout")
+                .help("Set the timeout for checkpoint sync calls to remote beacon node HTTP endpoint.")
+                .value_name("SECONDS")
+                .takes_value(true)
+                .default_value("60")
+        )
         .arg(
             Arg::with_name("reconstruct-historic-states")
                 .long("reconstruct-historic-states")

beacon_node/src/config.rs

@@ -441,6 +441,8 @@ pub fn get_config<E: EthSpec>(
                 .extend_from_slice(boot_nodes)
         }
     }
+    client_config.chain.checkpoint_sync_url_timeout =
+        clap_utils::parse_required::<u64>(cli_args, "checkpoint-sync-url-timeout")?;
 
     client_config.genesis = if let Some(genesis_state_bytes) =
         eth2_network_config.genesis_state_bytes.clone()