Add an onIcKeydown event to the searchable select

## Summary
Add an onIcKeydown event to the searchable select which is emitted when the user presses a key.

## 💬 Description
The text field component has an onIcKeydown event which a developer can use to determine what keys a user is pressing when the text field is focused, e.g. what characters are being inputted. It would be good to have the same event on a searchable select for the same reason.

## 💰 User value
It would make it more closely match the functionality available on the text field. Developers may want to use the information about which keys are being pressed to execute other functionality.

## Additional info
Based on a customer request (for a way to determine what keys have been pressed on the searchable select). `onIcInput` doesn't meet their requirements because they need to capture key codes like Control and Enter.

Locate the searchable select component and the text field implementation that already exposes onIcKeydown. Trace how the searchable select handles keyboard input, then add the corresponding event behavior so consumers can observe pressed keys, including Control and Enter; verify it with the component's existing tests.

Written by the indexing model from the issue text.