Introduces Objects Grid content block and block repetition mechanism migration

[bertyhell]

jira ticket:
https://meemoo.atlassian.net/browse/ARC-3792

example page:
http://localhost:3200/admin/content-paginas/af27aac8-40e9-4503-bf91-34892f49fc12

Adds the new ObjectsGrid content block, allowing users to display a dynamic grid of media objects:

• Configurable with an optional title, a HetArchief.be search URL, and up to three fixed (pinned) media objects.
• Displays a responsive grid that adjusts column count, tile height, and intelligently packs fixed objects with dynamic random results from the search query.
• Integrates with the search API by converting client-side search URLs into API request bodies via a new clientSearchUrlToApiSearchUrl configuration service.
• Includes comprehensive styling to handle different media types (video, audio, newspaper, image) and provide a polished user experience.

This pull request also initiates the migration of content blocks to a new, more robust repetition mechanism (from A to B):

• The BlockButtons component is refactored to use a fieldGroup for its repeatable elements, improving the content editor's configuration flexibility.
• Implements a lazy data migration strategy that automatically upgrades legacy array-based content block data to the new object-with-elements format on load, ensuring backward compatibility.
• Updates the content page editor's paste functionality to rehydrate and automatically migrate pasted blocks to the current configuration standard.