Add onboarding payment gateway setup methods

[joshuatf]

This PR adds the methods needed to allow setup of Paystack during onboarding.

Part of the remote payment gateways project ( woocommerce/woocommerce#32258 ).

Screenshots

Testing

1. Install and activate this plugin in your plugins/ directory matching the wp.org slug. woo-paystack
2. Install and activate the development version of WooCommerce Admin ( git clone https://github.com/woocommerce/woocommerce-admin.git && cd woocommerce-admin && composer install && npm i && npm run build ).
3. Make sure South Africa is selected as the store country under WooCommerce general settings.
4. Navigate to the Paystack gateway in the payments task /wp-admin/admin.php?page=wc-admin&task=payments
5. Fill out the required setting fields.
6. Make sure that the Paystack gateway is moved to the "Enabled payment gateways" section.
7. Make sure that settings are persisted on page refresh.
8. Make sure the currency is updated to ZAR after saving settings from the payment task.