feat(core): add missing fields to request structs

[MargaretKerubo]

Summary

This PR adds the missing data fields to the standard request structs in requests.go to match the specifications defined in the plan.md appendix.

Prior to this change, the structs only contained the IdempotencyKey. The new fields (like Amount, TransactionID) provide the necessary data schema to map provider-specific payloads (such as M-Pesa's STK push) to the unified Gateway interface.

Type of change

• Bug fix
• New feature
• Refactor
• Documentation

Related issue

Closes #61

Details

• PurchaseRequest & AuthorizeRequest: Added Amount, Currency, Phone, Description, CallbackURL, and Metadata fields.
• CaptureRequest & RefundRequest: Added TransactionID and Amount fields to reference the original transaction and support partial capture/refunds.
• VoidRequest & StatusRequest: Added TransactionID field.
• Documentation: Added Godoc comments to all exported structs explaining their specific purpose in the payment lifecycle.

Testing

• Ran mise run ci locally. Verified that the code compiles, golangci-lint passes without warnings, and all existing tests pass (go test ./...).

Checklist

• Tests added or updated
• Documentation updated
• go test ./... passes
• Breaking changes documented