Add ability to append any element on a EuiSideNav

[cauemarcondes]

On APM we needed to add a new badge beside the label on the observability nav. More details here: https://github.com/elastic/kibana/issues/107718

Currently, there's no way to automatically add such a badge on EuiSideNav. I would like to have a new property called append where it would be rendered beside the label, and it should also automatically truncate the label if it is bigger than the container.

I did a workaround to have it for 7.15 here https://github.com/elastic/kibana/pull/108397/files#diff-4d1ad7a610ef7e1d1282f54948f8331f0fe9effeedd43ce5ce70370501e87549R52

[cchaos]

The biggest challenges with work arounds are with the hover states underlining the appended content and keeping truncation.

Our own implementation in Docs is messy too: https://github.com/elastic/eui/blob/036a89c46e4e05923491b62b1e51d361ca08d0ec/src-docs/src/components/guide_page/_guide_page.scss#L45-L64

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[github-actions[bot]]

👋 Hey there. This issue hasn't had any activity for 180 days. We'll automatically close it if that trend continues for another week. If you feel this issue is still valid and needs attention please let us know with a comment.

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❌ We're automatically closing this issue due to lack of activity. Please comment if you feel this was done in error.