Allow nested EuiAccordions

[thompsongl]

Summary

Fixes #2129, in which an EuiAccordion with an EuiAccordion as a child element does not account for height change when the nested accordion opens.
More generally, EuiAccordion will now account for more than just nested instances, but any style mutation on children elements.
The solution involves abstracting a transition delay/duration service out of EuiPopover. These services work with MutationObserver to sum transition timings and run a callback using requestAnimationFrame

Checklist

- [ ] This was checked in mobile
- [ ] This was checked in IE11
- [ ] This was checked in dark mode
- [ ] Any props added have proper autodocs

• Documentation examples were added
• A changelog entry exists and is marked appropriately

- [ ] This was checked for breaking changes and labeled appropriately

- [ ] Jest tests were updated or added to match the most common scenarios

- [ ] This was checked against keyboard-only and screenreader scenarios
- [ ] This required updates to Framer X components

[thompsongl]

As far as I can tell, jsdom doesn't allow for setting the full gamut of style properties. CSSStyleDeclaration never gets the correct values for transition properties, even with browser prefixing. As such, I haven't found a way to test the added services, as they rely on reading CSSStyleDeclaration values.

[chandlerprall]

Played with some testing ideas, it would be far more work than worth it to test for a regression here.

LGTM; pulled & tested locally