Add Taxjar Calculator as dropdown option

[Noah-Silvera]

Currently you integrate this extension into your app by forcing it to be the default tax calculator

We should really create a named tax calculator for this extension and update the readme to instruct users to select the tax calculator from the calculators dropdown in the solidus admin

[Noah-Silvera]

This is still important for the user experience, but not required for certification, so it's lower priority.

There's also still some unknowns around how we would actually go about adding this to the dropdown - I don't think it's a trivial task from my vague memory of originally investigating this.