[EuiCard] Allow custom component for image prop

[TAYTS]

Summary

Issue: #3341
Added ReactElement to the EuiCard image prop to allow passing custom component.

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes

• Checked in mobile
  - [ ] Checked in IE11 and Firefox
• Props have proper autodocs
• Added documentation examples
  - [ ] Added or updated jest tests
  - [ ] Checked for breaking changes and labeled appropriately
  - [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[cchaos]

🎉 Thanks @TAYTS .

Just a could suggestions. Also, can you update one of the image cards to pass a custom element. It can just be as simple as:

<EuiCard
  image={<div><img src="...unsplash source url..." /></div>}
  ...rest of the props...
</EuiCard>

[cchaos]

J, test this

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3370/

[cchaos]

👍 Checked in IE, FF and Safari and the updated example still renders as it did. I'll just push a couple text updates then it will be good to merge! Thanks again @TAYTS

[cchaos]

retest