Add forward declaration for InternalIterator in memtable.h

[Wangch29]

Summary

This PR adds a forward declaration for InternalIterator in memtable.h to improve header self-containment and follow C++ best practices.

Problem

The memtable.h header file uses InternalIterator type in its interface (specifically in ReadOnlyMemTable::NewIterator() and ReadOnlyMemTable::NewTimestampStrippingIterator() methods) but doesn't provide a forward declaration or include the corresponding header file. This violates the self-contained header principle.

Solution

Added a forward declaration for InternalIterator in the forward declarations section of memtable.h.

Changes

• Added class InternalIterator; forward declaration in memtable.h

Impact

• Improves code maintainability: Makes dependencies explicit and clear
• Prevents potential compilation issues: Reduces risk of compilation failures due to missing includes
• Follows C++ best practices: Headers should be self-contained and include all necessary forward declarations
• No functional changes: This is a pure code quality improvement

Testing

• Existing tests should continue to pass
• No behavioral changes expected

Additional Notes

While the current code compiles successfully (likely due to indirect includes through other headers), adding the forward declaration makes the dependency explicit and prevents potential issues if the include chain changes in the future.

This is a preventive optimization that improves code quality without affecting functionality.