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>

Cargo.lock

@@ -3128,17 +3128,13 @@ dependencies = [
  "context_deserialize",
  "educe",
  "eip_3076",
- "either",
- "enr",
  "eth2_keystore",
  "ethereum_serde_utils",
  "ethereum_ssz",
  "ethereum_ssz_derive",
  "futures",
  "futures-util",
- "libp2p-identity",
  "mediatype",
- "multiaddr",
  "pretty_reqwest_error",
  "proto_array",
  "rand 0.9.2",