added UNIV2LP collateral

[hexonaut]

Description

Collateral Type (e.g. ETH-A): UNIV2LP

COB Team Name or Author(s): @hexonaut @lucas-manuel

GemJoin Adapter: GemJoin

Is this GemJoin adapter new? : No

If so, why is a new one needed? : N/A

Contribution Checklist

• Collateral token contract has been added in src/tokens/ as TOKEN.sol, if not already present
• If needed, a new GemJoin has been added in src/ as join-X.sol
• A new test (testGemJoinX_TOKEN()) has been added to src/join.t.sol and follows previous tests as examples
• If needed, other tests have been added to verify functionality of the new GemJoin:
  • testFailGemJoinXJoinWad()
  • testFailGemJoinXExitWad()
  • testFailGemJoinXJoin()
  • testFailGemJoinXExit()
  • testFailJoinAfterCageGemJoinX()
• README has been updated to reflect a new token and GemJoin addition

Checklist

• Confirm that token contract matches the verified source on etherscan mainnet
• If a new GemJoin is added, ensure the above description is accurate
• If a new GemJoin is added, ensure a new GemJoin has been added with the correct name
• Ensure README is updated
• Manually review the new test(s)
• Manually run dapp tests in src/join.t.sol to confirm they all pass

[gbalabasquer]

Please let's wait a bit on merging this PR as we still need to figure it out how we will do LPs in the scripts.