Add USDC as Accepted Collateral

This proposal adds Circle's USDC (issued on Stellar) as accepted collateral in the Quorum lending protocol. USDC would have an initial loan-to-value ratio of 90% and a liquidation threshold of 95%. USDC is one of the most liquid assets on the Stellar network. Its inclusion as collateral would meaningfully expand borrowing capacity for users who hold USDC. The Chainlink Stellar price feed for USDC/USD will be used for oracle pricing.