feat: add MULTIPLE_PAYMENTS option to BillingKind type

[luskafaria]

closes #17

Description

This PR extends the BillingKind type to include a new MULTIPLE_PAYMENTS option alongside the existing ONE_TIME option. This enhancement allows the SDK to support recurring or installment-based billing scenarios where a customer is charged multiple times for a service or product.

Changes

• Added MULTIPLE_PAYMENTS as a valid option in the BillingKind type definition;
• Made customer information optional when frequency is MULTIPLE_PAYMENTS;
• Added ACTIVE status to BillingStatus type;
• Updated IBilling type to include additional fields: allowCoupons, coupons, and couponsUsed;
• Modified customer field in IBilling to support empty objects for MULTIPLE_PAYMENTS;
• Created base types (BaseBillingData, BillingProduct) to reduce duplication;
• Split CreateBillingData into discrete types (OneTimeWithCustomerId, OneTimeWithCustomer, MultiplePayments).

[ChristoPy]

I like it! Just a comment hahaha

Also, we have a new version of our SDK, so remember to sync your branch when you get back to it.

See ya