⚡️ Derive Fee Swap Coin Out From IBC Fees Provided

[NotJeremyLiu]

Background

• Previously, the contract accepted a coin_out param in SwapExactCoinOut, but the logic would essentially ensure this coin_out provided matched the actual fees given in ibc_info.fee.
• So, since we made this enforcement, a simplification and safer flow from a caller would be such that the fee swap coin_out is directly derived from the provided ibc fees itself, leaving no way for the caller to specify an incorrect coin_out.

This PR

• Removes the coin_out field from SwapExactCoinOut
• Implements a one_coin method on IbcFee to ensure that if it provided it is exactly one coin (since the rest of the contract can't handle multiple ibc fee swaps anyways, and since it's an Option it should be set to None if no ibc fees are required)
• Changes the entry point contract to derive the fee swap coin out from the ibc fees
• Adds tests