Support Multiple Nexus Locations

Hello all!

After chatting to @juliaamosova & looking at some older issues (#1259 ) it looks like WCS only works with one location at this time. That means if your company has nexus in more than 1 location you can't use it.

Nexus can vary wildly based on your country, state, county, & municipal laws but some common examples of nexus are:

- A physical presence like an office, store front, or warehouse
- An employee
- Sometimes even an affiliate

So if I have an office in Denver and an employee in Littleton (and let's assume Littleton has a different municipal tax rate) I can't use Automated Tax feature in WooCommerce Services.

Similarly if I have a home office in Denver and I use a 3rd party fulfillment service (including Amazon) based in any other state I can't use the Automated Tax feature.

I'd love (and I think store owners would also love) if multiple locations for the purposes of tax nexus can be added to the service. :)

No files, tests, or entry points are named in the issue; start by locating the Automated Tax feature and its nexus-location handling. Done means store owners can configure multiple nexus locations, including differing jurisdictions, and use Automated Tax successfully for them.

Written by the indexing model from the issue text.