Add merge support to simulator (#3292)

## Issue Addressed

N/A

## Proposed Changes

Make simulator merge compatible. Adds a `--post_merge` flag to the eth1 simulator that enables a ttd and simulates the merge transition. Uses the `MockServer` in the execution layer test utils to simulate a dummy execution node.

Adds the merge transition simulation to CI.

beacon_node/beacon_chain/src/test_utils.rs

@@ -11,7 +11,9 @@ use crate::{
     StateSkipConfig,
 };
 use bls::get_withdrawal_credentials;
+use execution_layer::test_utils::DEFAULT_JWT_SECRET;
 use execution_layer::{
+    auth::JwtKey,
     test_utils::{ExecutionBlockGenerator, MockExecutionLayer, DEFAULT_TERMINAL_BLOCK},
     ExecutionLayer,
 };
@@ -361,6 +363,7 @@ where
             DEFAULT_TERMINAL_BLOCK,
             spec.terminal_block_hash,
             spec.terminal_block_hash_activation_epoch,
+            Some(JwtKey::from_slice(&DEFAULT_JWT_SECRET).unwrap()),
             None,
         );
         self.execution_layer = Some(mock.el.clone());