feat: Add GetDifferences method for ListItem field change detection

[jeppesc11]

Summary

Adds field change detection capabilities to ListItem with a new GetDifferences extension method that compares current and new field values,

Features:

• Comprehensive field type support: User, Lookup, Multi-lookup, Taxonomy, Geolocation, URL, DateTime, numeric types, and arrays
• Flexible null/empty handling: treatEmptyStringAsNull parameter for configurable empty string behavior
• Missing field handling: Treats non-existent fields as null values

Usage

var newValues = new Dictionary<string, object>
{
    ["Title"] = "Updated Title",
    ["AssignedTo"] = new FieldUserValue { LookupId = 5 },
    ["Categories"] = new[] { "Category1", "Category2" }
};

var differences = listItem.GetDifferences(newValues, treatEmptyStringAsNull: true);

foreach (var change in differences)
{
    Console.WriteLine($"Field '{change.FieldInternalName}' changed from '{change.CurrentValue}' to '{change.NewValue}'");
}

Breaking Changes

None - this is a new addition that doesn't affect existing functionality.