PR Description

This PR implements the feature described in GitHub issue #465: [Contract] Implement Granular Access Control and Delegation.

Summary

Added role-based access control with Role enum (Admin, Keeper, Delegate, Viewer, Auditor)
Added fine-grained permission system with Permission enum (TaskCreate, TaskExecute, TaskManage, PortfolioManage, GovernanceVote, GovernancePropose, KeeperRegister, KeeperDelegated, AdminAccess)
Implemented role assignment and revocation functionality
Implemented permission granting and revocation functionality
Implemented permission delegation and revocation functionality
Added new DataKey variants for storing role assignments, permission grants, and delegations
Added helper functions for managing role-based access control data

Added Role and Permission enums for defining roles and permissions
Added RoleAssignment, PermissionGrant, and Delegation structs for storing access control data
Added DataKey::RoleAssignments, DataKey::PermissionGrants, DataKey::Delegations, DataKey::RoleAssignmentCounter, DataKey::PermissionGrantCounter, and DataKey::DelegationCounter variants
Added helper functions get_role_assignment, set_role_assignment, get_permission_grant, set_permission_grant, get_delegation, set_delegation, and their counter variants
Added contract functions assign_role, revoke_role, grant_permission, revoke_permission, delegate_permission, and revoke_delegation
All functions include proper security guards and authorization checks
Added appropriate events for monitoring access control changes

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