[refactor] Implemention of default logger with json output

[knyk-dev]

Implement DefaultLogger in log/default_logger.go.

• It needs to implement the Logger interface.
• We should use RWMutex to ensure the enable switch is thread-safe. Don't use complex channels.
• For logic, OnBeforeEvent records start time. OnAfterEvent calcs duration, formats LogEntry to map, and prints as JSON string.
• It should support subscription filtering.
• Returning nil simply means log all events.

Tests:
Create log/default_logger_test.go. Verify the enabling switch and filter logic. Check if the output is valid JSON with expected fields. Ensure high test coverage and handle edge cases like nil pointers.

[hsluoyz]

done at: #1648