Retain className for EuiFormControlLayout.prepend nodes

[nreese]

Kibana maps application is using EuiFieldText to show the selected icon. This uses the prepend property like this. The problem is that the className property is not getting passed to the rendered prepend prop because EuiFormControlLayout over rights that prop to a static value. This PR just updates the logic to preserve the original className when provided.

        <EuiFieldText
          onClick={this._togglePopover}
          onKeyDown={this._handleKeyboardActivity}
          value={value}
          compressed
          readOnly
          fullWidth
          prepend={
            <SymbolIcon
              className="mapIconSelectSymbol__inputButton"
              symbolId={value}
              fill={isDarkMode ? 'rgb(223, 229, 239)' : 'rgb(52, 55, 65)'}
              stroke={isDarkMode ? 'rgb(255, 255, 255)' : 'rgb(0, 0, 0)'}
              strokeWidth={'1px'}
            />
          }
        />

[thompsongl]

LGTM!

Thanks, @nreese!

Can you add a changelog entry, also?