feat(settings): expose global charts context for settings

[nickofthyme]

Summary

adds ElasticChartsProvider to pass EchContext value to set globally shared values across all chart instances. This remediates the need to provide a theme and baseTheme to all chart instances, the theme will just be merged with any provided user theme, when the chart is wrapped in the provider.

export interface EchContext {
  settings?: SettingsProps;
}

used as...

<ElasticChartsProvider value={{ settings: mySettings }}>
  <Chart>
  <Settings /> {/* must provide spec to inherit from provider context */}
    {/* ...component specs */}
  </Chart>
</ElasticChartsProvider>

Details

Leaving the api for EchContext open for future props beyond just settings.

Issues

This completes a long ignored usage for shared global settings
fix #718

Depends on #1421

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-design 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

[nickofthyme]

We have discussed moving the eui theme into charts and providing a single theme. This could be useful to easily access the isDarkMode variable inside kibana.