Add aem-assets-contenthub-sample with card and selectionBar namespace…

[Naradasi-Tejaswi-3011]

Description

Adds a new sample app aem-assets-contenthub-sample for the aem/assets/contenthub/1 extension point, demonstrating all three supported namespaces in a single project:

• assetDetails — adds a custom tab panel to the Asset Details Dialog side rail (PanelAssetDetailsExtensionTab.js)
• card — adds a custom action button to asset cards and collection tiles; clicking it opens a modal (CardActionModal.js)
• selectionBar — adds a custom bulk action button to the selection bar when assets are selected; clicking it opens a modal (SelectionBarModal.js)

Checklist:

• I have signed the Adobe Open Source CLA.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.