feat(ui5-timeline): introduce header and info-bar slots

[hinzzx]

High-level Architecture additions

• Added a header slot for a controls bar above the items.
• Added an infoBar slot for a status bar below the controls, reflecting the active state.
• Added a stickyHeader boolean that pins the header bar to the top while scrolling.

Overview

The Timeline had no built-in way to place controls (search, filter, sort) above its items. Applications had to build custom layouts around it, leading to inconsistent patterns and no standardized sticky behavior.

What we did

Added two optional slots above the items area:

• header — for controls (search field, sort/filter buttons). Typically holds a ui5-toolbar.
• infoBar — for status display (active filter tokens, item count). Typically holds a ui5-bar.

Plus:

Property	What it does
stickyHeader	Pins both bars at the top while scrolling

Key points

• The Timeline does not search, sort, or filter — apps stay in full control.
• Sticky behavior is CSS-only — no JS measurement needed.
• Fully backwards compatible — Timelines without these slots work as before.

Test plan

• yarn test:cypress:single cypress/specs/Timeline.cy.tsx in packages/fiori
• Website samples render correctly (cd packages/website && yarn start)
• Search, filter, sort, and token removal work interactively
• Sticky behavior works when scrolling

[ui5-webcomponents-bot]

🧹 Preview deployment cleaned up: https://pr-13548--ui5-webcomponents.netlify.app

[sap-ui5-webcomponents-release]

🎉 This PR is included in version v2.24.0-rc.0 🎉

The release is available on v2.24.0-rc.0

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