[Lens][Accessibility] Take into account background color for non opaque colors

[dej611]

Summary

Fixes #106593

Color contrast now takes into account also EUI background color when computing final text color for the datatable dynamic coloring.

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)
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
• This was checked for cross-browser compatibility

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 86aa2ff
• Storybooks Preview

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
lens	1.6MB	1.6MB	+385.0B

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

[mbondyra]

Tested in the app and in canvas. Code LGTM.

[kibanamachine]

💚 Backport successful

Status	Branch	Result
✅	7.x

This backport PR will be merged automatically after passing CI.