Add updater for premium add-ons

[Remco Tolsma]

As mentioned in #308 we need an updater for our premium add-ons.

we only need a way to update some of the add-ons:

• https://github.com/pronamic/wp-pronamic-pay-adyen/blob/b3e3bb374066a4e9d0a83645325b11aca93ddd85/pronamic-pay-adyen.php#L31-L47
• https://github.com/pronamic/wp-pronamic-pay-paypal/blob/6045abc5eb25ee90a9b5c96dbc01ad1d1bd8bbf0/pronamic-pay-paypal.php#L36-L56
• https://github.com/pronamic/wp-pronamic-pay-payvision/blob/e2ec972590b49ccc541d54ffffa50b50b990e62e/pronamic-pay-payvision.php#L31-L47

As discussed via Slack we need a custom updater for this, similar to: https://github.com/pronamic/wp-pronamic-client/blob/develop/classes/Updater.php.

In the future we should include a Update URI: header in all the add-ons:
https://make.wordpress.org/core/2021/06/29/introducing-update-uri-plugin-header-in-wordpress-5-8/

[Remco Tolsma]

Added in dd9152f and pronamic/wp-pay-core@5d5eeac.