Add date format option with tests

[aydinomer00]

• Add Date struct with DateOnly flag
• Implement custom date formatting for Notion API
• Add comprehensive test coverage
• Fix timezone formatting in FormatForNotion
• Handle null values in UnmarshalJSON
• Closes #177

[wolveix]

@aydinomer00 this would be great to have :D Before @jomei has time to review it (hopefully soon?), here were my thoughts:

• I don't think it makes sense to move these into a new pkg/models directory when the rest of the repo doesn't already follow that structure
• Following the conventions of the rest of the library, date.go and date_object.go should be under one file