SelectableList: add optional key property to options prop

[mbondyra]

Summary

Fixes #2605

This is a proposed solution for fixing the React warning about duplicate keys for SelectableList if we pass two identical labels. The situation happens eg. for Index Pattern Chooser in Lens if index pattern was imported from Saved Object and the identically named index pattern existed in the app.

Adding optional prop 'key' for Option Interface would allow having two different objects with identical labels without throwing the warning.

Still, the PR doesn't fix the problem of differentiating two identically labelled objects from a user perspective.

Checklist

• Checked in dark mode
• 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

[cchaos]

Thanks @mbondyra for starting this PR.

You'll need to also add a changelog entry (last checklist item in the PR summary). You should be able to follow suit from the other items. Just make sure the entry it past tense.

[mbondyra]

@cchaos I added the changelog note. If it can be grammatically improved don't hesitate to add a suggestion 🙏

[cchaos]

Tested in Chrome, it works correctly. 👍

Just one more comment for the changelog.

[chandlerprall]

Changes LGTM; pulled and tested locally