ForwardRef support - Flex and Grid

[reesscot]

Issue:
Closes #710

Description of changes:
This PR adds forward ref support for Flex and Grid primitives by wrapping them in React.forwardRef. I'm limiting the number of primitives changed per PR to make them easier to review.

const CustomerComponent = () => {
  const ref = React.useRef(null);
  
  // do something with ref, like measure where it is on screen:
  // ref.current.getBoundingClientRect()
  
  return (
    <Flex ref={ref} />
  );
};

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[changeset-bot]

🦋 Changeset detected

Latest commit: c047f4d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@aws-amplify/ui-react	Patch

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[dbanksdesign]