[EuiComboBox] Add the loading spinner to input when isLoading

[shahzad31]

EuiComboxbox isLoading state is broken, when props isLoading is set to true, it doesn't do anything.

[cchaos]

The loading state is not actually broken. The current implementation of that component's loading state just replaces the dropdown contents with a loading indicator, so you have to actually open the combo box to see the loading state.

However, we could additionally add the loading spinner to the input as well similar to how the other inputs handle it.