[Discover] Add ownFocus to Discover field search popover for a11y

[kertal]

Summary

This PR improves Discover field search popover to be accessible by adding ownFocus to the EuiPopover. What doesn't work is adding focus to the first input element, seems to be an issue of EuiButtonGroup

So when you set the ownFocus to true but no initialFocus is the type Select is automatically focused:

When you set the ownFocus and try to set initialFocus to the first button group, no element is focused, but after using tab key the first EuiButtonGroup is focused:

Fixes #75190

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)
• Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
• This was checked for cross-browser compatibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 03169ec

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
discover	420.6KB	420.7KB	+93.0B

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[myasonik]

Being unable to successfully set initialFocus to EuiButtonGroup is definitely a bug (I saw you created elastic/eui#4266 which is perfect).

But leaving it off is also totally fine. Focus is moved to the popover container itself without initialFocus set which is a generally expected default. So you only need to set an initialFocus if you really want to.

So, if you want, you could leave it at that, merge the PR, and close the issue.

If you want initialFocus to be set to the EuiButtonGroup then you'll need to wait for EUI to fix the issue.

[myasonik]

Oh! Actually, you could close this PR all together as EUI recently merged a PR changing the default of ownFocus to true for all EuiPopovers which would close the issue. (Though this would still not setup an initialFocus.)

[kertal]

thx @myasonik, will close this PR since it doesn't make sense to add code, to later remove it.