Normalize tax breakdown attributes

[fastdivision]

Entity properties should be consistent across breakdown types to make it easier to drill down into tax breakdown responses. This PR makes a couple tweaks to reduce confusion. It's a breaking change that requires a major version update.

[codymullins]

@fastdivision I'd recommend we hold off on this until v4. We can make this cleaner for end users as well.

[CLAassistant]

All committers have signed the CLA.