[Contracts] Implement Global Yield Rate and Supported Token Registry Admin Controls

[ONEONUORA]

Description

Add admin/governance configuration functions:

• Let contract administrator update the global base yield rate.
• Add/remove supported assets (USDC, XLM, etc.).
• Update admin address.

Acceptance Criteria

• Restricts methods to admin signature only.
• Emits configuration change events.
• Unit tests validating state modification.

[ejioforJs]

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

Hello,i would like to work on this issue.

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

[BigNathan1]

@BigNathan1 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer, I would love to resolve this issue within few hours. I am a Full-Stack Dev with contributions to Oss projects. Please assign

ℹ️ Repo Maintainers: To accept this application, review their application or assign @BigNathan1 to this issue.

[drips-wave]

Congratulations, @BigNathan1! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @BigNathan1: 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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