Feat/admin/issuer - #163

closes #124

Completed: Delegated Admin Actions via Signed Off-Chain Authorization

Implemented support for delegated admin actions using signed off-chain authorization, allowing admin/issuer operations to be submitted by a relayer while preserving the existing direct authorization flow.

What was added

• Added a design writeup covering:

  • Signed payload structure ("target address", "action", "nonce", "expiry", and relevant contract context)
  • Signature verification flow
  • Replay protection using nonce tracking and expiry validation
  • Interaction with Soroban authorization patterns, including similarities with custom-account authorization models

• Implemented delegated allowlist update support as a proof of concept:

  • Added a delegated action entry point alongside the existing admin-authenticated path.
  • The relayer submits the transaction.
  • The contract verifies:
    • The signature was created by the registered admin/issuer.
    • The nonce has not already been consumed.
    • The authorization has not expired.
    • The requested action matches the signed payload.

Security considerations covered

• Prevented replay attacks through nonce consumption.
• Prevented stale authorizations through expiry checks.
• Rejected signatures from unauthorized keys.
• Preserved existing direct "require_auth" flows; delegated authorization is additive and does not replace current admin controls.

Tests added

Covered the delegated authorization lifecycle:

✅ Valid delegated action succeeds
✅ Replayed signature is rejected
✅ Expired authorization is rejected
✅ Signature from a non-admin key is rejected