[EuiSelectableTemplateSitewide] Should open popover when pressing `enter`

[cchaos]

I noticed a "bug" in Kibana and subsequently EUI's component, that when you escape to close the popover you can get it back unless you re-type. It's not a huge-workaround, but I would assume anytime your in a search box that hitting enter shows the results so I think we should just ensure the popup is visible on enter.

Here's a video, but you can't tell what keys I'm pressing.

Screen.Recording.2022-03-16.at.04.48.00.PM.mp4

[ki504178]

Hello!!
I will work on it.

[cee-chen]

Closed by #5886