[SC-07] Implement contract initialization and admin functions

[yusuftomilola]

Overview

Implement the contract initialization function and admin management functions inside contracts/contrib/.

Location

All work must be inside contracts/contrib/src/.

Acceptance Criteria

• initialize(env, admin: Address) — sets admin in storage under DataKey::Admin; panics if already initialized
• get_admin(env) -> Address — returns stored admin address
• add_registrar(env, registrar: Address) — admin only; adds address to authorized registrars list
• remove_registrar(env, registrar: Address) — admin only; removes address from registrars list
• is_authorized_registrar(env, address: Address) -> bool — returns true if address is a registrar
• get_total_asset_count(env) -> u64 — returns total registered asset count