feat: expose style for interpolation fit functions

[markov00]

Summary

This PR exposes the style options for the currently available interpolation fit functions.
In particular:

• the area path of an interpolated area can be customized as:

type AreaFitStyle = 
  Visible & // hide or show the area path
  Opacity & { // change the opacity used to fill the area
    fill: Color | typeof ColorVariant.Series;  // a specific color or the assigned data series color
    texture?: TexturedStyles; // an optional texture
  };

rendered with:

<AreaSeries
  areaSeriesStyle={{
    fit: {
      line: { visible: false },
      area: {
        fill: 'rgba(33, 33, 33, 0.3)',
        texture: { opacity: 0.2, stroke: 'gray', rotation: -45, shape: TextureShape.Line },
      },
    },
  }}

• the line path can be customized with:

type LineFitStyle =
  Visible & // hide or show the line path
  Opacity & // change the opacity used to render the line stroke
  StrokeDashArray & { // an optional dash stroke array
    stroke: Color | typeof ColorVariant.Series; // a specific color or the assigned data series color
  };

rendered with:

<LineSeries
    lineSeriesStyle={{
      fit: {
        line: {
          dash: [2, 10, 10, 5],
          stroke: 'rgba(0, 0, 0, 0.2)',
        },
      },
  }}

Details

The style is exposed through the LineSeriesStyle and the AreaSeriesStyle so it can be configured globally and/or configured per series.

I take the chance to refactor a bit the code, removing some optional arguments and reversing small logic to improve code readability.

cc @elastic/eui-design
Please double-check the theme property. We basically just expose the currently used style without breaking changes.
The visualization editor will take care of handling and applying a different style when requested by the user (generally they are just requesting to show a straight line and a filled area without any differences between the interpolated line/area and the original line/area created with the dataset.

Issues

fix #1489

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]

LGTM, I just made two comments on the api

[rshen91]

These changes LGTM - tested locally