♻️ Refactor Fee Swap Into Ibc Transfer Variant of Action Enum

[NotJeremyLiu]

Background

• Previously, the contract would throw an error if a fee_swap was provided but the post swap action was not an ibc transfer/
• This is unnecessary as the fee_swap option could instead be placed into the ibc transfer variant of the post swap action enum, enforcing this requirement via the types themselves.

This PR

• Refactors fee_swap by moving it into the Action enum's IbcTransfer variant
• Updates tests accordingly
• Changes entry point logic slightly to use an if let pattern instead of matching on the post swap action enum (since we only match on 1 variant now)