[EuiTab] Add option for secondary content like EuiNotificationBadge

[cchaos]

so one thing I noticed about this non-centering issue - I believe that when the button is hovered, an underline shows up both for the button copy and for the number within the EuiNotificationBadge, and the number appears centered when the underline is visible. I was unable to remove the underline with any of the usual css tricks - something to keep in mind!

Originally posted by @ecezalp in #4992 (comment)

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Yep because you're putting the badge inside of a button which gets text-decoration: underline on :hover. There's no way to separate out the button content vs non-button content from the consuming side. Usually you can add text-decoration: none !important to the part you don't want to get the attribute.

But I've also seen this pattern emerging more often (notification badge inside a tab) and maybe one we can implement as part of the EuiTab itself.

Originally posted by @cchaos in #4992 (comment)