Add woocommerce_my_account_edit_address_field_value filter hook and validate custom address type postcode field

1st commit: Allows developers to override the address field value before displaying it on the edit form. This can be useful when the address needs to be loaded from a different source than user meta (for example, an external API).

Note: There are at least 3 more places where this kind of filtering may be needed, but they are already covered by existing hooks:
* filtering the displayed formatted address on My account page can already be covered by the`myaccount_my_address_formatted_fields` filter hook
* filtering the address returned by clicking the "Load billing address" button on Edit Order view can be achieved by using the `woocommerce_found_customer_details` filter hook.
* filtering the value displayed in the checkout form can be achieved with `woocommerce_checkout_get_value` filter hook

2nd commit: Validates any field that ends with postcode as a postcode field, when processing edit-address form after submit. This change actually is something that was (accidentally) left out from one of my previous PRs.