Add SEP-10 client_domain verification step

[Haroldwonder]

The SEP-10 flow does not verify the client_domain claim in the challenge. Implement client domain verification by fetching the domain's stellar.toml and validating the signing key.

[ZeePearl56]

@ZeePearl56 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to work on this issue because I can get it done efficiently.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ZeePearl56 to this issue.

[drips-wave]

Congratulations, @micheamitchelle-rgb! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @micheamitchelle-rgb: 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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