Add custom client signature validation API

[kumaakh]

Implements a flexible client signature (host ID) validation system that allows applications to provide their own host identification logic instead of relying solely on licensecc's built-in hardware fingerprinting.

Key features:

• Added set_client_signature_validator() API function to register custom validators
• Custom validator receives the client signature string from the license file
• Validator returns boolean indicating whether the signature is valid
• One-time-only registration to prevent runtime tampering (locked after first set)
• Falls back to default licensecc hardware validation if no custom validator set
• Maintains backward compatibility - existing code works without changes

API additions:

• ClientSignatureValidator typedef for validator function signature
• set_client_signature_validator() function in licensecc.h public API
• Thread-safe implementation using static locked flag

Use cases:

• Cloud/virtualized environments where hardware IDs change
• Custom hardware identification schemes (e.g., USB dongles, TPM chips)
• Enterprise environments with centralized license servers
• Testing/development with mocked host validation

This feature enables more flexible licensing models while maintaining security through the one-time registration constraint.