Pool-based bridge model for canonical tokens. This model is intended for canonical tokens that have already been generated on different chains (e.g., USDT, USDC, ETH). When a canonical token needs to be transferred between chain A and chain B, two liquidity pools need to be first created on chain A and chain B, respectively. The bridge rate is dynamically adjusted according to the balances of the two liquidity pools, using the StableSwap pricing curve.