[SDK] Add verifyWebhookSignature helper and tests

[JayWebtech]

Overview

SDK should help merchants validate webhooks consistently.

Technical Requirements

• Implement HMAC verify matching backend signing format.
• Unit tests with fixtures.
• Export from package entry.

[dinahmaccodes]

@dinahmaccodes has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Hello, My name is Dinah and I would love to contribute. Please can you assign me one issue?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @dinahmaccodes to this issue.

[drips-wave]

Congratulations, @dinahmaccodes! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

🧑‍💻 @dinahmaccodes: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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