feat: implement universal rbac for contracts

[Agbasimere]

Closes: #25

Description

This PR implements a universal, modular Role-Based Access Control (RBAC) system for Soroban smart contracts. It provides a standard way to manage permissions (Admin, Moderator, Contributor) within the AnchorPoint ecosystem.

Key Changes

• Modular RBAC Module: Created src/auth/rbac.rs which can be used both as a library crate and as a standalone contract.
• Hierarchical Permissions:
  • Admin: Full control, can assign/revoke roles.
  • Moderator: Intermediate access level.
  • Contributor: Basic functionality level.
• Integrated Events: Automated event publication for role assignment and revocation to ensure a transparent audit trail on-chain.
• Unit testing: Comprehensive test suite covering initial setup, hierarchy validation, and authorization checks.

Integration

The module is designed for easy integration:

1. As a dependency: Add anchorpoint-auth = { path = "../auth" } to your contract's Cargo.toml.
2. As a contract: Deploy RBACContract as a centralized permission manager.

Commands

• Test: cargo test -p anchorpoint-auth

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[Ceejaytech25]

@Agbasimere , lgtm!