[EuiIconTip] Ensure aria-hidden attribute is suppressed to avoid axe (accessibility) errors in loading cycle

[1Copenut]

Is your feature request related to a problem? Please describe.
Our icon_tip.tsx file adds a tabindex=0 to the child EuiIcon on line 66. This is causing an accessibility violation in Kibana just now because icons with aria-hidden="true" have a tabindex that allows them to be reached by keyboard focus before the finished icon has been loaded.

Describe the solution you'd like
I'd like to find a way to signal to EuiIconTip that it should not be ARIA hidden by its nature. Maybe we could explicitly set the attribute undefined ?

<EuiToolTip position={position} delay={delay} {...rest}>
  <EuiIcon
+   aria-hidden={undefined}
    tabIndex={0}
    type={type}
    color={color}
    size={size}
    aria-label={ariaLabel}
    {...iconProps}
  />
</EuiToolTip>

Additional context
Kibana is tracking this error in elastic/kibana#172598. There may be others that haven't been discovered yet.

The Buildkite error logs showed this snippet, that suggests the scan is running while icons are not fully loaded:

<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16" class="euiIcon eui-alignTop css-1h8nuic-euiIcon-s-isLoading" tabindex="0" role="img" aria-label="Info" aria-hidden="true" data-is-loading="true"></svg>

[cee-chen]

Code to change:

eui/src/components/icon/icon.tsx

Line 299 in 17e0d61

// If it's an empty icon, or if there is no aria-label, aria-labelledby, or title it gets aria-hidden true

Here's my question for you Trevor - should we really be removing aria-hidden for empty/loading icons, or should we be removing the tabIndex="0"?

The latter feels more true to the intention of hiding icons that have no meaningful context attached, but the former feels more flexible for worst case scenarios, e.g.:

1. icon is unable to finish loading, due to e.g. internet issues
2. dev forgets to attach an aria-label but an aria-describedby does exist (thanks to the tooltip) and provides more helpful information than if it was completely hidden to SRs

[cee-chen]

I talked through this a bit more with @peteharverson in Slack and he helped me think through the above question a bit more. The conclusion we came to was that that not applying aria-hidden to empty icons makes the most sense if a label/title of some sort is being applied - we should always prioritize providing that label to screen reader users.