feat(legend): expose legend extra configuration

[markov00]

Summary

This PR rewrites and exposes the configuration for the legend extra parameter, now called legend value.

These are the changes available in the PR:

• replace the booleanshowLegendExtra prop in Settings with a configurable legendValue
• the legendValue prop now accept the following configurations:
  • none: don't display a value
  • lastBucket: works only

TODO.....

• renames of all extra and extra related functions, replacing the extra term with value or values. Extra wasn't a proper name and I feel that value is definitely generic but at least more specific
• adds a new storybook story to test the effectiveness of one possible value mode.
  Add an example of the wrong rendering of the last bucket in data.
  The example shows:
• A fixed xDomain that goes from May 16, 2023 @ 00:00:00.000 to May 17, 2023 @ 12:59:59.999 UTC+2
• The data is aggregated every 30 minutes
• The last bucket available is starts at May 17 at 12:30 and ends at May 17 at 12:59:59.999
• The chart shows an additional bucket from 13:00 to 13:30
• The legend doesn't show the last bucket value because of that additional bucket

Linked to elastic/kibana#153079 for a cleaner resolution of the problem

Details

Issues

Checklist

• The proper chart type label has been added (e.g. :xy, :partition)
• The proper feature labels have been added (e.g. :interactions, :axis)
• The :theme label has been added and the @elastic/eui-team team has been pinged when there are Theme API changes
• All related issues have been linked (i.e. closes #123, fixes #123)
• New public API exports have been added to packages/charts/src/index.ts
• Unit tests have been added or updated to match the most common scenarios
• The proper documentation and/or storybook story has been added or updated
• The code has been checked for cross-browser compatibility (Chrome, Firefox, Safari, Edge)
• Visual changes have been tested with all available themes including dark, light, eui-dark & eui-light

[markov00]

buildkite update screenshots

[markov00]

buildkite update screenshots

[markov00]

buildkite update screenshots

[elastic-datavis]

Deployments - 0817ed0

• Storybook (link)
• e2e server (link)
• Playwright report (link)