QSerializer Enhancements and Fixes

[DHR60]

Overview of Updates

This PR introduces several improvements to the QSerializer library, enhancing serialization capabilities and user experience. Main changes include:

1. Conditional definition support for QJsonDocument mode
2. Code structure optimization and warning reduction
3. Enhanced std::optional<T> JSON/XML serialization support
4. Empty value and null reference skipping functionality

Detailed Changes

Serialization Options Enhancement

• Implemented comprehensive skipping mechanism for empty values, null values, and null literals
• Added class-level and member-level serialization option controls
• Introduced shouldSkipMemberEmpty, shouldSkipMemberNull, and shouldSkipMemberNullLiterals methods for fine-grained control

Macro Definition Optimization

• Refactored QS_DERIVED_META_OBJECT_METHOD and related macros for code consistency
• Added QS_FIELD_DEFAULT and QS_OBJECT_DEFAULT macros to support default value setting
• Optimized QSERIALIZABLE macro definition

C++17 Compatibility

• Added C++17-based conditional compilation logic
• Utilized inline static member variables for improved implementation efficiency in C++17
• Maintained backward compatibility with older C++ standards

Optional Type Support

• Enhanced JSON and XML serialization handling for std::optional<T>
• Added dedicated macros for optional types: QS_JSON_FIELD_OPT, QS_XML_FIELD_OPT, etc.
• Implemented optional object type serialization: QS_JSON_OBJECT_OPT, QS_XML_OBJECT_OPT

Other Optimizations

• Converted QSerializer to a header-only INTERFACE library, improving ease of use
• Fixed Qt6 support issues
• Improved static function methods, including adding static fromJson functionality

Usage Example

// Using class-level settings to skip empty values and null
QS_CLASS_BEGIN(MySerializableClass)
    QS_INTERNAL_SKIP_EMPTY_AND_NULL
    
    QS_FIELD(QString, normalString)
    QS_FIELD_OPT(int, optionalInt)
    
    // Member-level settings
    QS_FIELD(QStringList, emptyListToSkip)
    QS_INTERNAL_MEMBER_SKIP_EMPTY(emptyListToSkip)
QS_CLASS_END

[DHR60]

In this PR, my primary focus is to introduce the QS_FIELD_DEFAULT macro and improve code structure optimization while reducing warnings, as these are key enhancements for the QSerializer library. Other changes, such as QS_FIELD_OPT and related optional type support macros (e.g., QS_FIELD_OPT, QS_INTERNAL_SKIP_EMPTY_AND_NULL), are part of my own requirements. If these optional type features are not needed by the project, I’m happy to open a separate PR to remove them.

Let me know if any adjustments are required!