Fix: oracle contract has no admin rotation mechanism

[yahia008]

Priority: High
Estimated Time: 1 hour

Description:
The oracle contract's admin address is set once at initialize and cannot be changed. If the oracle service key is compromised, there is no recovery path short of redeploying the contract.

Tasks:

• Add update_admin(env: Env, new_admin: Address) -> Result<(), Error> requiring current admin auth
• Update DataKey::Admin with the new address
• Add test for admin rotation
• Add test that old admin cannot call submit_result after rotation

[sheyman546]

@sheyman546 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I will love to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @sheyman546 to this issue.