Allow calling `setPostalCode` after the payment form has loaded

[mattallan]

Hi Square Team,
I'm using this package in our Square extension for WooCommerce and I need to be able to update the postal code field in the SqPaymentForm after the form has loaded (i.e. when the customer updates their postcode in the billing address section)

Normally I would just use paymentForm.setPostalCode() but this isn't exposed in the context.

Can you please look into adding a function like onSetPostalCode()? Similar to onCreateNonce and onVerifyBuyer?

Here's a screencast showcasing what I'm looking for: https://d.pr/v/8SliDF
Thanks