KeeperDAO Coordination facility for mStable AMM (0 Fees Swap for KeeperDAO keepers)

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Is your feature request related to a problem? Please describe.
Allow KeeperDAO keepers to arbitrage the AMM (mAsset). In order to give KeeperDAO an edge and to allow them to reach arbitrage price first, set fees to 0 for KeeperDAO keepers. Only whitelisted keepers can call the special function to bypass the fee.

Describe the solution you'd like

• Requires upgrading contracts
• adjust swap, redeem, mint, or add one function to figure out the path callable only for Keeper to set swapFee = 0 and pass it to the Logic (Library Contract).
• Add function to set a keeper
• Add modifier/or check in the function to check for keeper
  MassetLogic library to allow accept the swapFee from the calling function

Relevant contracts:
mStable-contracts/Masset.sol at master · mstable/mStable-contracts · GitHub mStable-contracts/MassetLogic.sol at master · mstable/mStable-contracts · GitHub

Additional context
More info: https://forum.mstable.org/t/rfc-explore-a-keeperdao-coordination-facility-for-mstable-amm/798/9

Pass Criteria

• Working contracts
• Unit tests
• Fork tests including the upgrade of contracts (Ethereum mainnet) and function testing.