[EuiDatePickerRange] endDateControl does not allow for overwriting popover placement

[dominiqueclarke]

EuiDatePicker allows overwriting popover placement through the popoverPlacement prop. However, when used with EuiDatePickerRange, the popover placement is overwritten and hardcoded to bottom-end. This prevents consumers from being able to specify popover placements relative to their app layout.

Reference: https://github.com/elastic/eui/blob/master/src/components/date_picker/date_picker_range.tsx#L96

Consumers should be able to overwrite the popover placement of EuiDatePicker, even when used as an end control for EuiDatePickerRange