feat: add DataIndex methods

[SantanaTiago]

This PR introduces two new methods to DataIndex class:

• list_items(); it allows user to list/search documents in a collection (also available in CLI)
• get_item_urls(); it allows user to get urls of a document to download, namely PDF, MD, and JSON

Changes:

• ElasticProjectDataCollectionSource location changed to elastic file to avoid circular imports

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