Refactor Wallet.fund method to be more re-usable

[danield9tqh]

Summary

For custom note selection we will want to implement different algorithms for funding a transaction. This refactor will make it easier to implement and swap out different funding methods

Testing Plan

Unit tests

Documentation

Does this change require any updates to the Iron Fish Docs (ex. the RPC API Reference)? If yes, link a related documentation pull request for the website.

[ ] Yes

Breaking Change

Is this a breaking change? If yes, add notes below on why this is breaking and what additional work is required, if any.

[ ] Yes