chore: drop-downs improvements and bug fixes

[aaryan610]

This PR includes bug fixes and improvements on drop-downs

Bug fixes:

1. Single select drop-downs not getting closed on selecting an option.
2. External classNames from props not overwriting existing classNames.
3. Drop-downs not getting auto-positioned in the list layout.

Improvements and Refactoring:

1. Created one common type for dropdown props.

export type TDropdownProps = {
  buttonClassName?: string;
  buttonContainerClassName?: string;
  buttonVariant: TButtonVariants;
  className?: string;
  disabled?: boolean;
  placement?: Placement;
  tabIndex?: number;
};

2. Updated the UI of the custom global drop-downs, including-

   • Custom menu.
   • Custom select.
   • Custom search select.

3. Replaced string interpolation for classNames with cn helper function.

4. Removed unnecessary props from custom global drop-downs.