Add Quest Metadata Hash Support

[Mkalbani]

Enables decentralized off-chain quest content storage and richer quest descriptions.

Acceptance Criteria

• Support storage of IPFS/Arweave metadata hashes.
• Metadata retrieval function should be exposed.
• Validation tests should cover empty and valid hashes.

[wokedi]

@wokedi has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi maintainer, i have gone through the task description and will like to work on this please. ETA: 6hours. Thank you

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

[drips-wave]

Congratulations, @wokedi! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @wokedi: 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.

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