(OUI Component Refresh) Combo Box- Refine search interactions by providing a prop to allow builders to select clear or return to last valid selection

[KrooshalUX]

Problem defined here: #95

Proposed solution:
As a user is typing, it is valid to present an error state when no match is found. However, if no selection is made because no value was selected, when the focus is lost from the input box, the value should be cleared or returned to the last valid selection. Provide a prop to builders to set this interaction (clear or returned). This would change the "default" behavior, requiring the builder to prescribe NOT clearing the value after a no-match search.