Expose tooltip z-index

[nickofthyme]

Is your feature request related to a problem? Please describe.
When using the tooltip with a position: fixed menu, the z-index context can conflict where the tooltip is above the menu. This should be exposed and set by default in eui theme.

Related to elastic/kibana#84178

Describe the solution you'd like
Tooltip z-index should be added to the charts theme object

Describe alternatives you've considered
Override the tooltip css directly

[markov00]

🎉 This issue has been resolved in version 24.5.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

Your semantic-release bot 📦🚀