feat(Form): enabled opt-in animations for expandable fieldgroup

[thatblindgeye]

What: Closes #11840

Additional issues:

[patternfly-build]

Preview: https://patternfly-react-pr-11843.surge.sh

A11y report: https://patternfly-react-pr-11843-a11y.surge.sh

[rebeccaalpert]

Spacing appears slightly off compared with the main site? I'm looking at the spacing above those dividers.

[nicolethoen]

Similar to my comment on TreeView - I think there are very valid reasons why a use case might incline a dev to want to dynamically create and render children of field groups. It may that they didn't load all the information from the backend on page load, and for performance reasons will only retrieve values for the collapsed field groups when they are expanded open... Something to keep in mind

[thatblindgeye]

Spacing appears slightly off compared with the main site? I'm looking at the spacing above those dividers.

@rebeccaalpert looks like the bug is also in Core when I copy+paste the React HTML into a Core example and revert the changes locally (other than bumping the Core versions). @srambach just FYI ^ in Rebecca's screenshot above there's a divider between "Nested field group 2 (non-expandable)" and the text input above it, but there's no spacing between the input and the divider.