Feat: native transfer fees tracking

[Jonathansumner]

Native Transfer Fees - Addressing #89

• Introduces saveNativeFeesEvent helper function
• Alters Transaction entity fees method to non-null list of Coin type
• implements rudimentary fee offset test within existing NativeBalanceChanges tests

(shoutout to @ejfitzgerald for the sage wisdom)

[bryanchriswhite]

Nailed it @Jonathansumner! 😄

Just that one thing we discussed: let's bypass the lookup for the -fee balance change for now and debug any errors we encounter where we're may be attempting to create multiple fee type balance changes for the same tx as this should never happen.

[bryanchriswhite]

Thanks @Jonathansumner! 😄