Implement Role-Based Access Control (RBAC) for Asset Management

[codebestia]

Summary

This PR implements comprehensive role-based access control across the AssetUp smart contracts, ensuring only authorized users can perform restricted actions like branch creation, asset tokenization, and audit logging.

Changes Made

• Enhanced existing admin storage and initialization in lib.rs
• Restricted create_branch to global admin only using require_auth()
• Restricted tokenize_asset to global admin only using require_auth()
• Created new audit_log.rs module with two access-controlled functions:
  • log_audit_action_as_owner: Only asset owners can log actions
  • log_audit_action_as_admin: Only global admin can log actions

🧪 Comprehensive Testing

• Created comprehensive tests verifying RBAC enforcement
• Verify authorized users can perform actions
• Verify unauthorized access is properly blocked
• Verify multiple audit logs can be created for the same asset

✅ Acceptance Criteria Met

• Only global admin can create branches and tokenize assets
• Only asset owners or global admin can log audit actions
• Tests confirm access control enforcement

Related Issue

Closes #252

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[codebestia]

GM @truthixify
Please review

[truthixify]

Thank you @codebestia