feat: add createExternalStore utility and ClockProvider

[gcutrini]

Add a generic external store factory (createExternalStore) using useSyncExternalStore that lets components subscribe to frequently-updating data sources without unnecessary re-renders.

Includes a pre-built clock store (ClockProvider, useClock, useClockSelector) wrapping the existing Clock component. This enables projects to move clock state out of Redux, eliminating per-second re-renders of all connected components.

API

• createExternalStore(name) → { Provider, useValue, useSelector }
• useValue() — re-renders on every emit
• useSelector(compute, isEqual) — re-renders only when the computed result changes

Clock usage

import { ClockProvider, useClock, useClockSelector } from 'openstack-uicore-foundation/lib/components/clock-context';

<ClockProvider timezone={summit.time_zone_id}>
  <App />
</ClockProvider>

const nowUtc = useClock(); // re-renders every second
const allowedTickets = useClockSelector(compute, isEqual); // re-renders only on change

ref: https://app.clickup.com/t/86b8dm4da