Epic: Collect and validate tax IDs for remaining countries

[haszari]

This epic covers all plugin & service changes to collect, validate and store tax IDs and business details as needed for remaining tax-docs countries.

Likely will spin off subtasks for different countries, and could break down further as needed.

Outcomes

• Merchant can supply tax ID & biz details as appropriate for country, using existing tax details modal.
• All new countries are supported: AU NZ NO SG
  • Check Taxamo settings for each country for messaging required when accepting Tax IDs.
  • See the Taxamo test account creds on secret store: Taxamo Transact (Test).
• The UK/GB tax validator is replaced by the Taxamo validator
• If any changes are needed for existing countries, e.g. essential refactoring, feature/devtest flag or UI improvements, that work is included in this epic.

Optional outcomes

These requirements are not essential to the project, but may be convenient to consider as part of this epic or as follow up. Note these are not reflected in estimate for this epic (21 SP), if they are implemented, please log issues/epics to track the estimate/SP for each bonus item.

• Store the validation date with the tax ID in database. This would allow us to re-validate periodically.
• Improve the naming of the metadata fields used to save tax info / biz details – e.g. add "tax" somewhere.
• Allow the merchant to provide tax ID / biz details any time, for example via a link/button in settings (or somewhere in admin).

See TSD: WooPayments tax-exclusive fees & tax (VAT/GST) documents for all countries