Add support for recurring payments via Adyen

[remcotolsma]

We had an internal Basecamp to-do for this since Jul 3, 2019:
https://basecamp.com/1810084/projects/10966871/todos/378755380

This feature was requested a few times since then:

• Jul 3, '19, 11:11 → https://secure.helpscout.net/conversation/894540297/17670
• Jun 12, '20, 12:22 → https://secure.helpscout.net/conversation/1191797646/19966/
• Apr 27, '21, 16:12 → https://secure.helpscout.net/conversation/1493477599/21984/
• Jul 15, '22, 15:54 → https://secure.helpscout.net/conversation/1950226687/24223

This is a bit in line with the following issues:

• Are recurring payments with Pay.nl supported? wp-pronamic-pay-pay-nl#10
• Are recurring payments with Google Pay supported? wp-pronamic-pay#143

Perhaps we should consider supporting recurring payments with another payment provider besides Mollie.

[rvdsteege]

Customer from Apr 27, '21 informed again about recurring payments support in internal Help Scout ticket https://secure.helpscout.net/conversation/1990595388/24378.