Ipv6 bootnodes (#3752)

## Issue Addressed
our bootnodes as of now support only ipv4. this makes it so that they support ipv6

## Proposed Changes
- Adds code necessary to update the bootnodes to run on dual stack nodes and therefore contact and store ipv6 nodes.
- Adds some metrics about connectivity type of stored peers. It might have been nice to see some metrics over the sessions but that feels out of scope right now.

## Additional Info
- some code quality improvements sneaked in since the changes seemed small
- I think it depends on the OS, but enabling mapped addresses on an ipv6 node without dual stack support enabled could fail silently, making these nodes effectively ipv6 only. In the future I'll probably change this to use two sockets, which should fail loudly

beacon_node/src/config.rs

@@ -14,6 +14,7 @@ use std::cmp::max;
 use std::fmt::Debug;
 use std::fmt::Write;
 use std::fs;
+use std::net::Ipv6Addr;
 use std::net::{IpAddr, Ipv4Addr, ToSocketAddrs};
 use std::path::{Path, PathBuf};
 use std::str::FromStr;
@@ -843,9 +844,11 @@ pub fn set_network_config(
     }
 
     if cli_args.is_present("enr-match") {
-        // set the enr address to localhost if the address is 0.0.0.0
-        if config.listen_address == "0.0.0.0".parse::<IpAddr>().expect("valid ip addr") {
-            config.enr_address = Some("127.0.0.1".parse::<IpAddr>().expect("valid ip addr"));
+        // set the enr address to localhost if the address is unspecified
+        if config.listen_address == IpAddr::V4(Ipv4Addr::UNSPECIFIED) {
+            config.enr_address = Some(IpAddr::V4(Ipv4Addr::LOCALHOST));
+        } else if config.listen_address == IpAddr::V6(Ipv6Addr::UNSPECIFIED) {
+            config.enr_address = Some(IpAddr::V6(Ipv6Addr::LOCALHOST));
         } else {
             config.enr_address = Some(config.listen_address);
         }