Support a way to mark an order tax exempt with hook_commerce_taxjar_tax_request_alter()

Problem/Motivation

From hook_commerce_taxjar_tax_request_alter() I don't have a clean way to have an order skip a call to TaxJar if it has been flagged as tax exempt in my system. The way I'm currently doing that is to pass a bogus 'to_country' so that it fails the tax nexus check and returns $0.

Proposed resolution

Add logic to support something like $request['tax_exempt_order'] that could be set in hook_commerce_taxjar_tax_request_alter() and would be checked at the same spot where there are nexus checks, so that one could conditionally bypass that call and get a $0 tax amount.