Implemented Multi-Signature Wallet Support

[Sandijigs]

Description

Enhanced the access control system with advanced multi-signature operations for critical administrative functions, significantly improving security for high-value operations.

fixes issue #425

Changes Made

• Enhanced Multisig Structures: Added tiered signature thresholds (standard, critical, emergency) with automatic proposal classification
• Time-Locked Operations: Implemented mandatory delay periods (24h default) for critical operations requiring multisig approval
• Emergency Procedures: Added emergency override mechanisms with higher signature thresholds (requiring all admins)
• Comprehensive Proposal Management: Full lifecycle support with approval, rejection, cancellation, and automatic expiration
• Signature Validation: Prevents duplicate approvals, validates thresholds, tracks rejections
• Proposal Cleanup: Automatic expiration after 7 days with manual cleanup capability

Files Modified

• contracts/access_control/src/types.rs - Added ProposalType enum, enhanced MultiSigConfig and PendingProposal structures
• contracts/access_control/src/errors.rs - Added 18 multisig-specific error codes
• contracts/access_control/src/access_control.rs - Implemented enhanced proposal workflow with 10+ new functions
• contracts/access_control/src/lib.rs - Added 9 new public endpoints
• contracts/access_control/src/access_control_tests.rs - Added 20+ comprehensive test cases
• contracts/manage_hub/src/lib.rs - Added integration documentation

Key Features

✅ Tiered signature thresholds (2/3/4 for standard/critical/emergency operations)
✅ Time-locked critical operations with configurable delays
✅ Emergency mode with highest approval requirements
✅ Proposal rejection mechanism with auto-rejection threshold
✅ Comprehensive statistics tracking (created/executed/rejected/expired)
✅ Automatic cleanup of expired proposals

Testing

• All code compiles successfully
• 20+ new test cases covering all scenarios
• Comprehensive validation of thresholds, time-locks, and workflows

Security Improvements

• Multi-admin consensus required for critical operations
• Time delays prevent hasty high-impact changes
• Emergency procedures require maximum oversight
• Duplicate approval prevention
• Automatic expiration prevents stale proposals

Example Usage

// Initialize with 5 admins, requiring 3 for standard ops
initialize_multisig(&env, admins, 3, None);
// Auto-calculates: critical_threshold=4, emergency_threshold=5

// Critical operation (needs 4 approvals + 24h delay)
let proposal_id = create_proposal(&env, admin1, 
    ProposalAction::UpdateConfig(new_config));

Closes #425

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