[EuiDataGrid] Introduce a renderCustomToolbar render prop

[kertal]

Summary

This PR is introducing a renderCustomToolbar render prop allowing the customize EuiDataGrid

QA

Remove or strikethrough items that do not apply to your PR.

General checklist

• Checked in both light and dark modes
• Checked in mobile
• Checked in Chrome, Safari, Edge, and Firefox
• Props have proper autodocs (using @default if default values are missing) and playground toggles
• Added documentation
• Checked Code Sandbox works for any docs examples
• Added or updated jest and cypress tests
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• Updated the Figma library counterpart
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_7150/

[elasticmachine]

💔 Build Failed

• Buildkite Build
• Commit: 1b407ad

Failed CI Steps

• 🔨 EUI pull request test

[cee-chen]

@kertal @timductive What happened here? Why did this PR get merged in with failing CI checks, missing QA checklists, and zero review from the EUI team?

If y'all want to take over datagrid and responsibility for its maintenance - awesome. Let's talk about moving it out of the EUI codebase first.

[cee-chen]

Ah, wait, I see - this merged into a feature branch and not into main. Apologies.

This generally isn't the way we prefer to handle feature branches. If each individual PR into the feature branch is thoroughly and atomically reviewed/QA'd, the final (usually large) feature branch merge doesn't need more than a quick confidence/smoke check.

Can you let us know what the plan is for this work and how you'd like us to work with you on it? I was planning on reviewing Matthias's PR and am happy to work with y'all on that, but if you'd prefer something else, happy to chat about it.

[kertal]

@cee-chen Ooops, sorry for the confusion. The issue @timductive was working on was labeled as "help wanted". The issue became redundant with our conversation to use renderCustomToolbar @timductive. However parts of Tims code could be used also in the renderCustomToolbar work, so I asked him to create a PR for it, and I wanted to push a commit to it, applying your suggestion quickly (but I couldn't, that's why I created a PR to his branch), which he merged. This happened right before the weekend, so because the next task from my side would have been informing you, hey we've a Draft, how to precede?

And it seems both out way creating PRs were creating branches on EUI and not on our forks, which should be done differently ideally.

Happy to have a chat with you how we can bring this over the finishing line 👍