Ensure difficulty/hash/epoch overrides change the ChainSpec (#2798)

* Unify loading of eth2_network_config

* Apply overrides at lighthouse binary level

* Remove duplicate override values

* Add merge values to existing net configs

* Make override flags global

* Add merge fields to testing config

* Add one to TTD

* Fix failing engine tests

* Fix test compile error

* Remove TTD flags

* Move get_eth2_network_config

* Fix warn

* Address review comments

beacon_node/client/src/builder.rs

@@ -148,19 +148,10 @@ where
             None
         };
 
-        let terminal_total_difficulty = config
-            .terminal_total_difficulty_override
-            .unwrap_or(spec.terminal_total_difficulty);
-        let terminal_block_hash = config
-            .terminal_block_hash_override
-            .unwrap_or(spec.terminal_block_hash);
-
         let execution_layer = if let Some(execution_endpoints) = config.execution_endpoints {
             let context = runtime_context.service_context("exec".into());
             let execution_layer = ExecutionLayer::from_urls(
                 execution_endpoints,
-                terminal_total_difficulty,
-                terminal_block_hash,
                 config.fee_recipient,
                 context.executor.clone(),
                 context.log().clone(),