Allow EuiListGroupItem to pass a custom element as the icon

[cchaos]

This is required for the nav in Kibana where plugins can provide their own (non-EuiIcon icon type) icon/image.

Example using an avatar:

Also beefed up the test cases.

Checklist

• This was checked in mobile
• This was checked in IE11
• This was checked in dark mode
• Any props added have proper autodocs
• [ ] Documentation examples were added I specifically didn't add an example as it's a very low use case
• A changelog entry exists and is marked appropriately
• [ ] This was checked for breaking changes and labeled appropriately
• Jest tests were updated or added to match the most common scenarios
• [ ] This was checked against keyboard-only and screenreader scenarios
• [ ] This required updates to Framer X components

[chandlerprall]

changes LGTM!

[snide]

Approved. Will add an issue to show this use case in the docs, which we likely want to have so we can accurately monitor it works :)