feat(Dropdown): Added new props for Dropdown - additionalSelectProps, popperClassName

[konstista]

Added several new props to Dropdown API:

• additionalSelectProps - supports custom props from downshift API, for example: initialIsOpen or onStateChange
• popperClassName - assigns a classname to <ul> element inside Dropdown component

Checklist

• The implementation has been manually tested and complies with Textkernel browser support guidelines
• The implementation complies with accessibility standards.
• The component has a displayName defined.
• The component comes with a detailed PropTypes (and defaultProps) definition.
• Component PropTypes are sufficiently described / documented.
• There is a story in Storybook.