Reduce eth2 dependency space (#8524)

Remove certain dependencies from `eth2`, and feature-gate others which are only used by certain endpoints.

| Removed | Optional | Dev only |
| -------- | -------- | -------- |
| `either` `enr` `libp2p-identity` `multiaddr` | `protoarray` `eth2_keystore` `eip_3076` `zeroize` `reqwest-eventsource` `futures` `futures-util` | `rand` `test_random_derive` |

This is done by adding an `events` feature which enables the events endpoint and its associated dependencies.
The `lighthouse` feature also enables its associated dependencies making them optional.

The networking-adjacent dependencies were removed by just having certain fields use a `String` instead of an explicit network type. This means the user should handle conversion at the call site instead. This is a bit spicy, but I believe `PeerId`, `Enr` and `Multiaddr` are easily converted to and from `String`s so I think it's fine and reduces our dependency space by a lot. The alternative is to feature gate these types behind a `network` feature instead.


Co-Authored-By: Mac L <mjladson@pm.me>

beacon_node/http_api/src/lib.rs

@@ -2118,9 +2118,12 @@ pub fn serve<T: BeaconChainTypes>(
                     let discovery_addresses = enr.multiaddr_p2p_udp();
                     Ok(api_types::GenericResponse::from(api_types::IdentityData {
                         peer_id: network_globals.local_peer_id().to_base58(),
-                        enr,
-                        p2p_addresses,
-                        discovery_addresses,
+                        enr: enr.to_base64(),
+                        p2p_addresses: p2p_addresses.iter().map(|a| a.to_string()).collect(),
+                        discovery_addresses: discovery_addresses
+                            .iter()
+                            .map(|a| a.to_string())
+                            .collect(),
                         metadata: utils::from_meta_data::<T::EthSpec>(
                             &network_globals.local_metadata,
                             &chain.spec,