Altair networking (#2300)

## Issue Addressed

Resolves #2278 

## Proposed Changes

Implements the networking components for the Altair hard fork https://github.com/ethereum/eth2.0-specs/blob/dev/specs/altair/p2p-interface.md

## Additional Info

This PR acts as the base branch for networking changes and tracks https://github.com/sigp/lighthouse/pull/2279 . Changes to gossip, rpc and discovery can be separate PRs to be merged here for ease of review.

Co-authored-by: realbigsean <seananderson33@gmail.com>

beacon_node/eth2_libp2p/src/rpc/methods.rs

@@ -1,6 +1,6 @@
 //! Available RPC methods types and ids.
 
-use crate::types::EnrBitfield;
+use crate::types::{EnrAttestationBitfield, EnrSyncCommitteeBitfield};
 use regex::bytes::Regex;
 use serde::Serialize;
 use ssz_derive::{Decode, Encode};
@@ -10,6 +10,7 @@ use ssz_types::{
 };
 use std::ops::Deref;
 use strum::AsStaticStr;
+use superstruct::superstruct;
 use types::{Epoch, EthSpec, Hash256, SignedBeaconBlock, Slot};
 
 /// Maximum number of blocks in a single request.
@@ -93,13 +94,23 @@ pub struct Ping {
 }
 
 /// The METADATA response structure.
-#[derive(Encode, Decode, Clone, Debug, PartialEq, Serialize)]
+#[superstruct(
+    variants(V1, V2),
+    variant_attributes(
+        derive(Encode, Decode, Clone, Debug, PartialEq, Serialize),
+        serde(bound = "T: EthSpec", deny_unknown_fields),
+    )
+)]
+#[derive(Clone, Debug, PartialEq, Serialize, Encode)]
 #[serde(bound = "T: EthSpec")]
 pub struct MetaData<T: EthSpec> {
     /// A sequential counter indicating when data gets modified.
     pub seq_number: u64,
-    /// The persistent subnet bitfield.
-    pub attnets: EnrBitfield<T>,
+    /// The persistent attestation subnet bitfield.
+    pub attnets: EnrAttestationBitfield<T>,
+    /// The persistent sync committee bitfield.
+    #[superstruct(only(V2))]
+    pub syncnets: EnrSyncCommitteeBitfield<T>,
 }
 
 /// The reason given for a `Goodbye` message.
@@ -360,7 +371,7 @@ impl<T: EthSpec> std::fmt::Display for RPCResponse<T> {
                 write!(f, "BlocksByRoot: Block slot: {}", block.slot())
             }
             RPCResponse::Pong(ping) => write!(f, "Pong: {}", ping.data),
-            RPCResponse::MetaData(metadata) => write!(f, "Metadata: {}", metadata.seq_number),
+            RPCResponse::MetaData(metadata) => write!(f, "Metadata: {}", metadata.seq_number()),
         }
     }
 }