beacon_node: add --disable-deposit-contract-sync flag (#3597)

Overrides any previous option that enables the eth1 service.
Useful for operating a `light` beacon node.

Co-authored-by: Michael Sproul <micsproul@gmail.com>

beacon_node/client/src/builder.rs

@@ -457,7 +457,9 @@ where
             ClientGenesis::FromStore => builder.resume_from_db().map(|v| (v, None))?,
         };
 
-        self.eth1_service = eth1_service_option;
+        if config.sync_eth1_chain {
+            self.eth1_service = eth1_service_option;
+        }
         self.beacon_chain_builder = Some(beacon_chain_builder);
         Ok(self)
     }