feat: add PaystackAdapter with payment, webhook, and refund support

[codefromlani]

Part of #38

This PR adds the Paystack adapter for EasySwitch, implementing full support for:

• Payment initialization (send_payment)
• Transaction status checks (check_status)
• Refund processing (refund)
• Webhook validation and parsing (validate_webhook, parse_webhook)
• Transaction detail retrieval (get_transaction_detail)
• Proper handling for unsupported operations (e.g., cancel_transaction)

Additionally, unit tests are included to cover:

• Webhook signature validation
• Payment initialization
• Transaction status retrieval
• Cancellation error handling

All tests pass and the adapter is fully compatible with EasySwitch’s transaction validation and type system.

Tests:

[codefromlani]

@Einswilli
Implemented all suggested changes, parse_webhook now returns WebhookEvent, send_payment uses format_transaction, get_transaction_detail returns TransactionDetail, and get_normalize_status has been added for Paystack status mapping

[Einswilli]

First of all, Great Work!
Can we have a full usage example with detailed explanation for the doc under docs/assets/examples/paystack.md??? (maybe in a new PR)

[codefromlani]

First of all, Great Work! Can we have a full usage example with detailed explanation for the doc under docs/assets/examples/paystack.md??? (maybe in a new PR)

Thanks. Sure