Add USDT as a new collateral type

[usmanert]

Description

Contribution Checklist

• PR title starts with (PE-<TICKET_NUMBER>)
• Code approved
• Tests approved
• CI Tests pass

Checklist

• Every contract variable/method declared as public/external private/internal
• Consider if this PR needs the officeHours modifier override
• Verify expiration (30 days unless otherwise specified)
• Verify hash in the description matches here
• Validate all addresses used are in Goerli changelog or known
• Notify any external teams affected by the spell so they have the opportunity to review
• Deploy spell to Goerli ETH_GAS_LIMIT="XXX" ETH_GAS_PRICE="YYY" make deploy
• Ensure contract is verified on Goerli etherscan
• Change test to use Goerli spell address and deploy timestamp
• Cast spell on Goerli make spell="0x-deployed-spell-address" cast-spell
• Run make archive-spell or make date="YYYY-MM-DD" archive-spell to make an archive directory and copy DssSpell.sol, DssSpell.t.sol and DssSpell.t.base.sol
• squash and merge this PR

[DaiFoundation-DevOps]

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