[EuiAvatar] Adding support for iconType, iconSize, and iconColor

[cchaos]

EuiAvatar used to only display either initials or a static background image. This PR adds support for displaying an EuiIcon.

It updates the props to the exclusion type of either initials, imageUrl OR iconType. The avatar will continue to color and size the icon based on the color and size props of the avatar but you can override this with adjacent props of iconSize and iconColor specifically.

Disabled:

It also adds one named color plain matching the name of EuiPanel's for simply a white background, and the ability to completely remove the background color with null which would allow consumer's to more easily create dark/light theme colors with classNames and SASS.

This PR also breaks down the docs page a little better with more sections detailing specifically the types and disabled.

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in Chrome, Safari, Edge, and Firefox
• Props have proper autodocs and playground toggles
• Added documentation
• Checked Code Sandbox works for the any docs examples
• Added or updated jest tests
• [ ] Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4620/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4620/

[thompsongl]

Code changes looking good. The test failure is due to a valid(?) axe a11y issue:

14:12:45 Errors on tp://localhost:9999/#/display/avatar
14:13:44 [heading-order]: Ensures the order of headings is semantically correct
14:13:44 Help: https://dequeuniversity.com/rules/axe/4.1/heading-order?application=axe-puppeteer
14:13:44 Elements:
14:13:44 - #types > div:nth-child(2) > div:nth-child(2) > h4
14:13:44 1 accessibility errors

"Spaces" h4 is the one in question.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4620/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4620/

[thompsongl]

LGTM with just some minor type tightening.

The imageUrl change is less impactful right now, but requiring iconType when that config section is in use will prevent setting other icon-related props when no icon is set.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4620/

[cchaos]

I'm going to merge this in once it all goes green.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4620/