contracts-bedrock: L2StandardBridge `initialize()`

Adds an `initialize()` function to the `L2StandardBridge`
implementation. This standardizes on having noop init functions
for L2 contracts. They are set in genesis state directly so the initialize
function should never execute. Keeping it there is more transparent.
Otherwise one may forget that it is initializable. The `_initialized`
storage slot MUST be set during L2 genesis generation.

packages/contracts-bedrock/snapshots/abi/L2StandardBridge.json

@@ -523,6 +523,13 @@
     "stateMutability": "payable",
     "type": "function"
   },
+  {
+    "inputs": [],
+    "name": "initialize",
+    "outputs": [],
+    "stateMutability": "nonpayable",
+    "type": "function"
+  },
   {
     "inputs": [],
     "name": "l1TokenBridge",

packages/contracts-bedrock/src/L2/L2StandardBridge.sol

@@ -61,6 +61,10 @@ contract L2StandardBridge is StandardBridge, ISemver {
         StandardBridge(payable(Predeploys.L2_CROSS_DOMAIN_MESSENGER), _otherBridge)
     { }
 
+    /// @notice Initializes the contract. This is a noop in the implementation but included to ensure that
+    ///         the contract cannot be initialized a second time.
+    function initialize() public initializer { }
+
     /// @notice Allows EOAs to bridge ETH by sending directly to the bridge.
     receive() external payable override onlyEOA {
         _initiateWithdrawal(