feat(ui): support native HTML attributes on Flex component

Extend FlexProps to accept standard HTML div attributes (e.g. onClick,
id, aria-*) by inheriting from React.HTMLAttributes<HTMLDivElement>.
The rest props are spread onto the underlying div element.

Signed-off-by: Johan Persson <johanopersson@gmail.com>

.changeset/flex-html-attributes.md

@@ -0,0 +1,7 @@
+---
+'@backstage/ui': patch
+---
+
+Added support for native HTML div attributes on the `Flex` component.
+
+**Affected components:** Flex