Feature/admin role rotation

[Abrahamojobo]

Closes #124

What changed

• Storage: Added ProposedAdmin to types.rs DataKey to track pending administrative transfer requests.
• Events: Added AdminTransferredEvent to emit a trace whenever the role is safely rotated.
• Error Handling: Added NotProposedAdmin to errors.rs to enforce strict security checks during acceptance.
• Implementation: Replaced statically assigned initialization-only admin logic with a robust, two-step rotation flow in lib.rs:
  • transfer_admin(new_admin): Requires the current admin's signature to securely cast a proposal.
  • accept_admin(): Requires the new admin's signature to execute the swap and clean up the proposal, eradicating the risk of locking the contract due to typographical errors.
• Testing: Added rigorous MockAuth test coverage mimicking Soroban SDK v25 security traps to guarantee unauthorized proposals or acceptances fail organically.