feat: implement robust Role-Based Access Control (RBAC) system

[Abdul-dev-creator]

RBAC Implementation for Issue #22

Description

This PR addresses Issue #22 by implementing a comprehensive Role-Based Access Control (RBAC) system and Emergency Pause mechanism for the StellarGrant contracts. It ensures fine-grained permissions for all core administrative and operational functions.

Key Implementation Details:

• New RBAC Module: Created access_control.rs to modularize all authorization and pause logic.
• Defined Core Roles: Implemented Admin, GrantCreator, Reviewer, and Pauser roles.
• Emergency Pause System: Integrated a contract-wide pause/unpause mechanism triggered by authorized roles (Pauser can pause, Admin can unpause).
• Function Protection: Added require_role and require_unpaused guards to all state-changing functions, including:
  • grant_create, grant_create_high_security
  • cancel_grant, grant_complete
  • milestone_vote, milestone_reject
  • grant_fund, fund_batch
  • sign_release, stake_to_review, unstake, slash_reviewer
• Persistent Storage: Securely stored roles and pause status using unique Soroban storage keys.
• Event Logging: Emitted RoleGranted, RoleRevoked, and PauseStateChanged events for auditability.
• Unit Testing: Added a full suite of tests in rbac_test.rs to verify both success and unauthorized access rejection paths.

Requirements Fulfilled:

• RBAC implementation module added.
• Integration with existing grant functions (e.g., only Admin/Owner can cancel).
• Emitted events for role changes.
• Comprehensive unit tests (for success and unauthorized paths).
• Documentation updated in README.md (initialization steps).

Closes #22

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[Samuel1505]

@Abdul-dev-creator kindly resolve merge conflicts