BTI-978: add In3 route param for ABN-AMRO Achteraf Betalen

[vildanbina]

• Add support for ABN-AMRO "Achteraf Betalen" (Op rekening – pay after 30 days) through the existing In3 (V3) flow.
• Allow an optional route service parameter on the In3 builder; sending route=abn_b2b switches the acquirer to ABN-AMRO. Plain In3 requests are unchanged.
• Add an example script under examples/ demonstrating the payment.
• Covered by unit tests: updated allowed-params snapshot plus new tests asserting route is optional, survives validation, and is absent when unused.

Jira: BTI-978