Simplify enum -> str with strum (#2164)

## Issue Addressed

NA

## Proposed Changes

As per #2100, uses derives from the sturm library to implement AsRef<str> and AsStaticRef to easily get str values from enums without creating new Strings. Furthermore unifies all attestation error counter into one IntCounterVec vector.

These works are originally by @blacktemplar, I've just created this PR so I can resolve some merge conflicts.

## Additional Info

NA


Co-authored-by: blacktemplar <blacktemplar@a1.net>

beacon_node/eth2_libp2p/src/peer_manager/peer_info.rs

@@ -11,6 +11,7 @@ use serde::{
 use std::collections::HashSet;
 use std::net::{IpAddr, SocketAddr};
 use std::time::Instant;
+use strum::AsRefStr;
 use types::{EthSpec, SubnetId};
 use PeerConnectionStatus::*;
 
@@ -320,21 +321,13 @@ impl Default for PeerStatus {
 }
 
 /// Connection Direction of connection.
-#[derive(Debug, Clone, Serialize)]
+#[derive(Debug, Clone, Serialize, AsRefStr)]
+#[strum(serialize_all = "snake_case")]
 pub enum ConnectionDirection {
     Incoming,
     Outgoing,
 }
 
-impl ConnectionDirection {
-    pub fn as_static_str(&self) -> &'static str {
-        match self {
-            ConnectionDirection::Incoming => "incoming",
-            ConnectionDirection::Outgoing => "outgoing",
-        }
-    }
-}
-
 /// Connection Status of the peer.
 #[derive(Debug, Clone)]
 pub enum PeerConnectionStatus {