Support for latest License Manager Server Update

Server Manager Updates:

OOP approach for managing license, security enhancement, update meta key generation (commit b07dd51).

• SECURITY ENHANCEMENT: Forbid direct REST API access. Applied filter hook to disable it, if not needed.
• ADDED: Strict check for product slug on the request parameter. Without it, the request fails.
• ADDED: Typehints.
• UPDATED: Meta key is now generated using product slug and client URL. This allows for multiple client managers on the same site. Old metadata is upgraded automatically.
• UPDATED: Error in REST API response data.
• UPDATED: Request error for activating or deactivating route.
• UPDATED: Changed action hook to filter hook for validating the request.
• REMOVED: Transient usage.
• REMOVED: Unnecessary error message with data when validate license happening.
• FIXED: Default option value if an empty string was saved.
• REVIEW: Refactor codes and comments.