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>

common/eth2/src/error.rs

@@ -14,6 +14,7 @@ use std::{fmt, path::PathBuf};
 pub enum Error {
     /// The `reqwest` client raised an error.
     HttpClient(PrettyReqwestError),
+    #[cfg(feature = "events")]
     /// The `reqwest_eventsource` client raised an error.
     SseClient(Box<reqwest_eventsource::Error>),
     /// The server returned an error message where the body was able to be parsed.
@@ -91,6 +92,7 @@ impl Error {
     pub fn status(&self) -> Option<StatusCode> {
         match self {
             Error::HttpClient(error) => error.inner().status(),
+            #[cfg(feature = "events")]
             Error::SseClient(error) => {
                 if let reqwest_eventsource::Error::InvalidStatusCode(status, _) = error.as_ref() {
                     Some(*status)