feat: add withdraw method to relay contract (#585)

contracts/teller-network/EscrowRelay.sol

@@ -72,6 +72,16 @@ contract EscrowRelay is RelayRecipient, Ownable {
     setRelayHub(IRelayHub(_relayHub));
   }
 
+  /**
+   * @notice Withdraw the ETH used for relay trxs
+   * @dev Only contract owner can execute this function
+   */
+  function withdraw() external onlyOwner {
+    IRelayHub rh = IRelayHub(getHubAddr());
+    uint balance = getRecipientBalance();
+    rh.withdraw(balance, msg.sender);
+  }
+
   /**
    * @notice Determine if the timeout for relaying a create/cancel transaction has passed
    * @param _account Account to verify