[EuiComboBox] Icons and buttons not correctly sized in append/prepend

[BenAOlson]

Combo boxes are given height: auto instead of the normal fixed height for regular text fields, and as such, the height: 100% value for the append/prepend children does not size icons or buttons to the full height (with plain text being the 'kind-of' exception as it has padding that brings the element to the correct height).

Code sandbox

[cchaos]

Thanks for the catch @BenAOlson!