[12.x] Add methods for managing TaxIDs

[driesvints]

This PR adds some methods to easily manage TaxID's on a customer. This is useful for keeping this data in Stripe so it also appears on its invoices an later in our own receipts like in #1136

Usage:

// Return all of the customer's TaxIDs in an collection instance...
$user->taxIds(); 

// Retrieve a specific TaxID from the customer...
$user->findTaxId('txi_IzcmyhZukd4oKD');

// Create a new TaxID for the customer...
$user->createTaxId($type, $value);

// Delete a TaxID from the customer...
$user->deleteTaxId('txi_IzcmyhZukd4oKD');

Closes #1071

[driesvints]

This was apparently referencing an invalid exception. Seems like no-one has hit this snag so far...