[EuiAutoSizer] Migrate from class to function component

`EuiAutoSizer` is an empty subclass of `react-virtualized-auto-sizer`'s `AutoSizer`. It exists to re-export the component under EUI's namespace.

- **File:** `components/auto_sizer/auto_sizer.tsx`
- **Class size:** 1 line (empty body)
- **Lifecycle:** Inherited from `AutoSizer`

**Migration notes:**

This depends on whether `react-virtualized-auto-sizer` itself supports function component usage or provides hooks. If the upstream library still exports a class, this wrapper may need to remain or be replaced with a hook-based alternative. Investigate upstream API first.

**Acceptance criteria:**

- [ ] Investigate upstream `react-virtualized-auto-sizer` API
- [ ] Convert to function component or re-export appropriately
- [ ] All existing tests pass