Shift networking configuration (#4426)

## Issue Addressed
Addresses [#4401](https://github.com/sigp/lighthouse/issues/4401)

## Proposed Changes
Shift some constants into ```ChainSpec``` and remove the constant values from code space.

## Additional Info

I mostly used ```MainnetEthSpec::default_spec()``` for getting ```ChainSpec```. I wonder Did I make a mistake about that.


Co-authored-by: armaganyildirak <armaganyildirak@gmail.com>
Co-authored-by: Paul Hauner <paul@paulhauner.com>
Co-authored-by: Age Manning <Age@AgeManning.com>
Co-authored-by: Diva M <divma@protonmail.com>

beacon_node/src/config.rs

@@ -581,8 +581,10 @@ pub fn get_config<E: EthSpec>(
         };
     }
 
-    client_config.chain.max_network_size =
-        lighthouse_network::gossip_max_size(spec.bellatrix_fork_epoch.is_some());
+    client_config.chain.max_network_size = lighthouse_network::gossip_max_size(
+        spec.bellatrix_fork_epoch.is_some(),
+        spec.gossip_max_size as usize,
+    );
 
     if cli_args.is_present("slasher") {
         let slasher_dir = if let Some(slasher_dir) = cli_args.value_of("slasher-dir") {