List Economic Nexus Thresholds

Returns the economic nexus thresholds that decide when a seller has to register in a state: the sales figure, the transaction count where one still applies, how the two combine, which sales count toward them, whether marketplace sales are included, and the period each state measures over. This is reference data. It is the same for every caller and is not derived from your sales: compare it against your own totals to work out where you are approaching a threshold. Send no body, or no `stateCodes`, to get every jurisdiction the country publishes. `countryCode` defaults to USA, currently the only country published. States with no general statewide sales tax (Delaware, Montana, New Hampshire, Oregon) are returned with `noSalesTax: true` and null thresholds rather than omitted. A `stateCodes` list naming a code that is not published is rejected with 400 rather than answered with the states that did match, so a typo cannot read back as 'this state has no threshold'.