Support for API versions; gem should upgrade before it is forced

[pdg137]

According to https://developers.taxjar.com/api/reference/?ruby#api-version , there are various versions of the API. By email we were told that the version will be force-upgraded for everyone on August 31, but there's no obvious way to test the latest version with this gem.

It seems to me that the gem should provide access to the version and also default to the latest version of the API starting now, so that anyone can test it out as part of their normal gem upgrade process.

[bray]

@pdg137 With the latest gem version (3.0.3), I'm able to set the API version like so:

TaxjarClient.set_api_config('headers', {
  'x-api-version' => '2022-01-24'
})

I agree though that this should be in the README and default to the latest API version.