Add a lookup function for UK VAT numbers

This should use the new version of the API that requires an API key.

It should also have some unit test to check some known values.

This should be similar to the existing lookup functions in src/business/datasource

https://developer.service.hmrc.gov.uk/api-documentation/docs/api/service/vat-registered-companies-api/1.0/oas/page#tag/organisations/operation/GetaVATregistration