WordPress REST API endpoints for gateways, payment methods and extensions

[remcotolsma]

https://github.com/pronamic/wp-pronamic-pay/blob/6.9.6/pronamic-ideal.php

• /wp-json/pronamic-pay/v1/gateways
• /wp-json/pronamic-pay/v1/gateways/mollie
• /wp-json/pronamic-pay/v1/gateways/mollie/methods
• /wp-json/pronamic-pay/v1/extensions
• /wp-json/pronamic-pay/v1/extensions/woocommerce
• /wp-json/pronamic-pay/v1/methods
• /wp-json/pronamic-pay/v1/methods/ideal

Can be useful for our own marketing websites.

Pending #154.

[remcotolsma]

In #231 (comment) and pronamic/wp-pay-core@6717591 we introduced the /wp-json/pronamic-pay/v1/gateways/:gateway_id endpoint to retrieve a functional gateway based on a gateway configuration post ID. We therefore have to look for another endpoint for a REST API endpoint for purely the gateway integrations that we support. We (@rvdsteege and I) talked about /wp-json/pronamic-pay/v1/integrations/gateways.