Preserve x-axis order in vislib.

[lukeelmers]

Closes #31534.

Summary

This resolves an issue where the original x values sort order sent back
by ES was lost for point series / vislib visualizations. This
was due to the way the point series agg response handler generated
series data, only filling in series values as it encountered them
bucket-by-bucket, rather than first looking at all x-values and ordering
them consistently within each series.

With this change, the ordered x values are preserved and stored with the
chart, so they can be accessed later in vislib/components/zero_injection.

QA

1. Create a stacked bar chart using kibana_sample_data_logs
2. Set time range to today
3. Metrics:
   • Y-Axis: Count
4. Buckets:
   • X-Axis: terms on extension.keyword, order: alphabetical ascending, size: 5
   • Split Series: terms on machine.os.keyword, order: count descending, size: 3
5. The values along the x-axis should be in alphabetical order (they will appear out of order if you test against master).

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

- [ ] This was checked for cross-browser compatibility, including a check against IE11
- [ ] 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
- [ ] This was checked for keyboard-only and screenreader accessibility

• Unit or functional tests were updated or added to match the most common scenarios

[elasticmachine]

Pinging @elastic/kibana-app

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[ppisljar]

works on chrome linux
i have some minor comments/questions around code

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[markov00]

Code LGTM, tested locally together with Peter and fix the ordering

[ppisljar]

LGTM