Added the ability to add custom classes to the main elements

[gnbm]

• This PR aims to add new customizations that can be used to close Add classes to button #312

What was happening

• The community was asking for a way to add CSS classes to some key elements that would allow the customization of the dropdown elements using utility classes (tailwind, bootstrap, ... )

After the fix

• The following options were added:
  • additionalDropboxClasses - additional classes for dropbox element
  • additionalDropboxContainerClasses - additional classes for the dropbox container element
  • additionalToggleButtonClasses - additional classes for the toggle button element
• A new documentation section called Custom styling was added to better showcase the usage of these new options:

• Followed the same steps above - ✅
• Ran regression scenarios - ✅
• Run automated tests - ✅