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/src/proposer_duties.rs

@@ -3,7 +3,7 @@
 use crate::state_id::StateId;
 use beacon_chain::{
     beacon_proposer_cache::{compute_proposer_duties_from_head, ensure_state_is_in_epoch},
-    BeaconChain, BeaconChainError, BeaconChainTypes, MAXIMUM_GOSSIP_CLOCK_DISPARITY,
+    BeaconChain, BeaconChainError, BeaconChainTypes,
 };
 use eth2::types::{self as api_types};
 use safe_arith::SafeArith;
@@ -33,7 +33,7 @@ pub fn proposer_duties<T: BeaconChainTypes>(
     // will equal `current_epoch + 1`
     let tolerant_current_epoch = chain
         .slot_clock
-        .now_with_future_tolerance(MAXIMUM_GOSSIP_CLOCK_DISPARITY)
+        .now_with_future_tolerance(chain.spec.maximum_gossip_clock_disparity())
         .ok_or_else(|| warp_utils::reject::custom_server_error("unable to read slot clock".into()))?
         .epoch(T::EthSpec::slots_per_epoch());