BTI-715: support optional In3 CompanyName & CocNumber params

[vildanbina]

• In3 accepts two optional B2B billingCustomer fields, CompanyName and CocNumber. They already flow through the SDK's billingCustomer list-group; no SDK code change was needed.
• Adds a feature test asserting both fields reach the wire on the Pay flow as GroupType=Billingcustomer, GroupID=1.
• Adds the two fields to the Pay demo in the In3 example so the usage is discoverable (BTI-637).
• Verified against the Buckaroo test gateway: no parameter errors for these fields; the gateway accepts them.