Add config options for broken down business address (This probably replaces the invoicing address config option I added recently).

In any tax field allow input of either:
- float number
- "EU" (gets standard rate from https://euvat.ga/rates.json)
- "TIC: <number>" (uses TaxCloud)

There must be field validation for that.

The eCommerce hooks would fill in the 'tax' field by looking up the tax for if the customer is buying from the business address.
However it would also carry a 'tax_code' field, and the correct tax would be worked out after the customer address is entered, and carried through.

I think TaxCloud will also tell us details of the exact taxes charged. We need to carry this through all the way to the transaction log. For EU taxes we should say the country it is for. This should be structured as a list of taxes. For the CSV import we should have each one be its own column, so it can be tallied.

This should all be documented.

form_input_tax form input element, that has separate radio button + field for each tax input type
post_param_tax function to read in tax value and verify
have a tax field type instead of list for catalogues, uses form_input_tax (so no pre-defined tax rates, except the written category descriptors that TaxCloud provides); should default to the most common category descriptor, assuming taxcloud is enabled