Is your feature request related to a problem? Please describe.
EuiInlineEdit is currently a self-contained component that allows consumers to set a defaultValue, but currently no way to update or control that value without un-mounting and remounting the component. The editMode value within the text form control can be manipulated with editModeProps, but the readModeValue that appears in the empty button cannot be modified at all.

Describe the solution you'd like
Create an optional value prop that accepts a string. When the value prop is passed, it will take precedence over the inline edit internal readMode value and any placeholder passed to the component. It should also be passed as the starting value for the editMode form control.