Use V2 of the UK VAT API and allow adding credentials

[chrisrollins65]

No description provided.

[bakaat]

I'd need to try this to be honest, but I'd suggest handling lookup service configuration, authentication and auth caching within the package. Sure it might take a bit longer but we're talking about major package version change anyway as we're either changing the Validate interface or the flow.

Personally I'd suggest to try providing additional "Configure" function if we want to keep the current structure. Flow would change to something like

vat.Configure(&ValidatorOptions{...}) 

or 

vat.Configure([]LookupServiceInterface{NewUKLookupService{username, pass, etc.}, NewVEISLookupService})

// after that calling Validate would mean it goes through configured services
Validate(vatNumber) {
  if vatNumber.HasPrefix(GB) {
    if lookupService := vat.LookupServiceManager.get("GB") {
      return lookupService.Validate(vatNumber)
    } else { 
      return LookupServiceNotConfigured{} 
    }
  }
}

alternatively ditch the single Validate function being exposed and provide a NewVat() with ability to configure validators, stop using the package directly. Initialise and pass where need instead.

[bakaat]

Discussed directly. We can look at the whole package design later on. For now we'll just keep it simple and get rid of the errors we're getting atm.

[chrisrollins65]

This all sounds good, would probably need some sort of cache interface setup and what not. As @bakaat mentioned, we can use the changes in this PR as a "fix" for the UK issue in this version and we can implement those other types of changes in the next version (v4) of this package.

[bakaat]

lgtm

[bakaat]

Discussed directly. We can look at the whole package design later on. For now we'll just keep it simple and get rid of the errors we're getting atm.