Add support for tax rate estimation

The module currently only supports shipping rate estimation. Let's look into appropriate tax rate estimation support as well.

This is tricky, though, as the expected behavior is more likely that we update the order total area handler on the cart form to show taxes inline. This would require not just an estimation of taxes via a temporarily edited order object like we use for shipping rate estimation but a full update of all line items on the order as well.

If you begin work on this, please outline your proposal for discussion first so we can review the user experience together.