Add PYUSD to the asset registry

Aquarius governance is voting to whitelist PYUSD, a 1:1 USD-backed stablecoin issued by Paxos Trust Company on behalf of PayPal. With 7.9M tokens already in circulation and a Stellar Expert rating of 7.5/10, PYUSD represents a major liquidity option for the ecosystem.

Whitelisting PYUSD expands stablecoin diversity on Stellar. The protocol currently relies heavily on USDC; adding Paxos's offering provides users a second major USD-backed option and reduces single-issuer concentration risk for Aquarius.