feat: implement admin role management and access control system

[CollinsC1O]

Admin Role Management and Access Control

Closes #23

Summary

Implemented a comprehensive admin role management system with secure access control for privileged contract operations including verification approvals, treasury access, and fee management.

Changes

New Module: admin_manager.rs

• Admin initialization, addition, and removal functionality
• Admin status checking and listing
• Protection against removing the last admin
• Event emission for admin changes

Updated Modules

• storage_keys.rs: Added Admin(Address) and AdminList storage keys
• errors.rs: Added CannotRemoveLastAdmin and AdminNotFound error types
• events.rs: Added publish_admin_added_event and publish_admin_removed_event
• lib.rs: Exposed admin management functions in contract interface
• verification_registry.rs: Enforced admin-only access for approve/reject operations
• fee_manager.rs: Enforced admin-only access for fee configuration

Access Control

All privileged operations now require admin authorization:

• ✅ Verification approval/rejection
• ✅ Fee configuration
• ✅ Admin management (add/remove)

Safety Features

• Cannot remove the last admin (prevents lockout)
• All admin operations emit events for transparency
• Comprehensive authorization checks with descriptive error messages

Contract compiles successfully

Testing

• 13 unit tests covering all admin functionality
• Integration tests for admin-protected operations
• Tests for unauthorized access attempts
• All tests passing ✅

New Public Functions

pub fn initialize(env: Env, admin: Address)
pub fn add_admin(env: Env, caller: Address, new_admin: Address)
pub fn remove_admin(env: Env, caller: Address, admin_to_remove: Address)
pub fn is_admin(env: Env, address: Address) -> bool
pub fn get_admin_list(env: Env) -> Vec<Address>
pub fn get_admin_count(env: Env) -> u32

[Naomi-Gift]

@CollinsC1O please resolve conflicts
pull and push again