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/http_api/tests/tests.rs

@@ -1,7 +1,7 @@
 use beacon_chain::test_utils::RelativeSyncCommittee;
 use beacon_chain::{
     test_utils::{AttestationStrategy, BeaconChainHarness, BlockStrategy, EphemeralHarnessType},
-    BeaconChain, StateSkipConfig, WhenSlotSkipped, MAXIMUM_GOSSIP_CLOCK_DISPARITY,
+    BeaconChain, StateSkipConfig, WhenSlotSkipped,
 };
 use environment::null_logger;
 use eth2::{
@@ -2313,7 +2313,9 @@ impl ApiTester {
             .unwrap();
 
         self.chain.slot_clock.set_current_time(
-            current_epoch_start - MAXIMUM_GOSSIP_CLOCK_DISPARITY - Duration::from_millis(1),
+            current_epoch_start
+                - self.chain.spec.maximum_gossip_clock_disparity()
+                - Duration::from_millis(1),
         );
 
         let dependent_root = self
@@ -2350,9 +2352,9 @@ impl ApiTester {
             "should not get attester duties outside of tolerance"
         );
 
-        self.chain
-            .slot_clock
-            .set_current_time(current_epoch_start - MAXIMUM_GOSSIP_CLOCK_DISPARITY);
+        self.chain.slot_clock.set_current_time(
+            current_epoch_start - self.chain.spec.maximum_gossip_clock_disparity(),
+        );
 
         self.client
             .get_validator_duties_proposer(current_epoch)