Add placeholder option for dropdown

[leeejune]

General Support Request

Add options to toggle placeholder and label on and off in Figma.

As a user I want to have the option of having to no placeholder by default so that the user can implement just a label like we can in input.

Possible Solution

Have options for:
label=true
placeholder=true,
label=false
placeholder=false,
label=false,
placeholder=true.

[leeejune]

Blueprint updated: https://www.figma.com/file/3TQhiKjQqQOKYqaqcO1zj2/Dropdown?node-id=906%3A753

@jason-capsule42 Please review!

[braven112]

Review what's happing with placeholder between dropdown and dropdownmenu

[Patrick-Daly-AA]

NOTE: we would like to have a wholistic approach and ensure a unified set of uses across select, input, datepicker, and combobox. Implementation of that will be handled in a subsequent issue.

[Patrick-Daly-AA]

Make sure to outline the user experience for when to display placeholder text based on element state, as shown in the linked figma doc, especially as we look into the broader solution across all our components.