feat: USDS migration for xDAI bridge

[zengzengzenghuy]

What

Background: https://forum.gnosis.io/t/gip-118-should-sdai-be-replaced-by-susds-in-the-bridge/9354

How

1. New Router contract(BridgeRouter.sol) to route relayTokens function and claim token executeSignatures(to receive DAI) / executeSignaturesUSDS(to receive USDS) to xDAI bridge, safeExecuteSignaturesWithAutoGasLimit to AMB/Omnibridge.
   1. Router contract is controlled by bridge governors and upgradeable
   2. Routes:
      1. DAI → XDaiBridgePeripheral
      2. USDS → XDaiBridge
      3. ETH →WETHOmnibridgeRouter
      4. Other tokens → Omnibridge
2. xDAI Foreign bridge only accepts deposit from USDS token.
   1. Changing the address of daiToken() to USDS and sDaiToken() to sUSDS.
   2. calling XDaiForeignBridge's executeSignatures and BridgeRouter's executeSignatures will always receive DAI
   3. calling XDaiForeignBridge's executeSignaturesUSDS and BridgeRouter's executeSignaturesUSDS will receive USDS post upgrade, while BridgeRouter's executeSignaturesUSDS will revert pre upgrade.
   4. XDaiForeignBridge's swapSDAIToUSDS() is a one time function that is called during the upgrade and changes the collateral from DAI to USDS.

New contracts

1. BridgeRouter: https://github.com/gnosischain/tokenbridge-contracts/blob/feat/xdai-usds-migration/contracts/upgradeable_contracts/erc20_to_native/BridgeRouter.sol , Upgradeable with TransparentUpgradeableProxy https://github.com/gnosischain/tokenbridge-contracts/blob/feat/xdai-usds-migration/contracts/upgradeability/TransparentUpgradeableProxy.sol
2. XDaiBridgePeripheral: https://github.com/gnosischain/tokenbridge-contracts/blob/feat/xdai-usds-migration/contracts/upgradeable_contracts/erc20_to_native/XDaiBridgePeripheral.sol
3. XDaiBridgePeripheralForDaiPreUsdsUpgrade: https://github.com/gnosischain/tokenbridge-contracts/blob/feat/xdai-usds-migration/contracts/upgradeable_contracts/erc20_to_native/XDaiBridgePeripheralForDaiPreUsdsUpgrade.sol
4. XDaiBridgePeripheralForUsdsPreUsdsUpgrade: https://github.com/gnosischain/tokenbridge-contracts/blob/feat/xdai-usds-migration/contracts/upgradeable_contracts/erc20_to_native/XDaiBridgePeripheralForUsdsPreUsdsUpgrade.sol

Modified contracts

1. XDaiForeignBridge: https://github.com/gnosischain/tokenbridge-contracts/blob/feat/xdai-usds-migration/contracts/upgradeable_contracts/erc20_to_native/XDaiForeignBridge.sol
2. SavingsDaiConnector: https://github.com/gnosischain/tokenbridge-contracts/blob/feat/xdai-usds-migration/contracts/upgradeable_contracts/erc20_to_native/SavingsDaiConnector.sol

Fork Test

https://github.com/gnosischain/tokenbridge-contracts/blob/feat/xdai-usds-migration/test/foundry/forkTest.sh

Bridge validator test (Check if bridge validator continues to work after bridge migration & xDAI can be still be minted on Gnosis Chain)

https://github.com/gnosischain/xdai-bridge-test

Reference

1. How xDAI bridge works: https://docs.gnosischain.com/bridges/About%20Token%20Bridges/xdai-bridge