Add support for Billie payment method

[rvdsteege]

Billie is the leading B2B Buy Now, Pay Later payment method in Germany. With Billie, business buyers get flexible payment terms while merchants receive a guaranteed payout.

Currently, Billie is only available for Mollie customers residing in Germany.

https://www.mollie.com/payments/billie

[remcotolsma]

Added in:

• https://github.com/pronamic/wp-pay-core/commit/6308ab6efc6af243fe882a07e5cf4193249984f9
• https://github.com/pronamic/wp-mollie/commit/a2b6e5846b6aaf19db661959c58860e9b804ec1b
• https://github.com/pronamic/wp-pronamic-pay-mollie/commit/59ea0a8911cbe5521c4102343c53e58963ced6ab
• https://github.com/pronamic/wp-pronamic-pay-woocommerce/commit/40f83854b4130133dbab9980330ba9c82e7f9195
• https://github.com/pronamic/wp-pay-logos/commit/90b60ac63e7bbd25fbc5c47f39a65f76a23b556e