Update _modifyLiquidity in BaseCustomCurve to use IPoolManager.ModifyLiquidityParams

[cairoeth]

🧐 Motivation
Currently, the BaseCustomCurve uses it's own encoding for modifyParams in _modifyLiquidity. However, ideally it uses the same struct as in BaseCustomAccounting so that there's mode code overlap and allows the possibility of adding more explicit checks in the add/removeLiquidity functions (such as checking that poolManager.modifyLiquidity matches the function type -- i.e. positive for add liquidity, negative for remove liquidity)

📝 Details
N/A