Vat checker validation extension

[syjer]

Add a hook point for handling the validation in case/additionally to the EU one.

Ideally, there can be multiple validation extension: if one of them return true, the vat nr is considered valid.

Flow:

1. user enter vat nr
2. if EU -> validate with vies -> if ok -> consider ok
3. if false OR not eu -> use all registered extensions and if one of them return true -> consider ok.

Parameters provided to the extension:

• vat nr
• country
• ???