Eth1 network exit on wrong network id (#1563)

## Issue Addressed

Fixes #1509 

## Proposed Changes

Exit the beacon node if the eth1 endpoint points to an invalid eth1 network. Check the network id before every eth1 cache update and display an error log if the network id has changed to an invalid one.

beacon_node/eth1/src/http.rs

@@ -12,8 +12,10 @@
 
 use futures::future::TryFutureExt;
 use reqwest::{header::CONTENT_TYPE, ClientBuilder, StatusCode};
+use serde::{Deserialize, Serialize};
 use serde_json::{json, Value};
 use std::ops::Range;
+use std::str::FromStr;
 use std::time::Duration;
 use types::Hash256;
 
@@ -30,6 +32,40 @@ pub const DEPOSIT_COUNT_RESPONSE_BYTES: usize = 96;
 /// Number of bytes in deposit contract deposit root (value only).
 pub const DEPOSIT_ROOT_BYTES: usize = 32;
 
+#[derive(Debug, PartialEq, Clone, Serialize, Deserialize)]
+pub enum Eth1NetworkId {
+    Goerli,
+    Mainnet,
+    Custom(u64),
+}
+
+impl FromStr for Eth1NetworkId {
+    type Err = String;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            "1" => Ok(Eth1NetworkId::Mainnet),
+            "5" => Ok(Eth1NetworkId::Goerli),
+            custom => {
+                let network_id = u64::from_str_radix(custom, 10)
+                    .map_err(|e| format!("Failed to parse eth1 network id {}", e))?;
+                Ok(Eth1NetworkId::Custom(network_id))
+            }
+        }
+    }
+}
+
+/// Get the eth1 network id of the given endpoint.
+pub async fn get_network_id(endpoint: &str, timeout: Duration) -> Result<Eth1NetworkId, String> {
+    let response_body = send_rpc_request(endpoint, "net_version", json!([]), timeout).await?;
+    Eth1NetworkId::from_str(
+        response_result(&response_body)?
+            .ok_or_else(|| "No result was returned for block number".to_string())?
+            .as_str()
+            .ok_or_else(|| "Data was not string")?,
+    )
+}
+
 #[derive(Debug, PartialEq, Clone)]
 pub struct Block {
     pub hash: Hash256,