Implement diff_file_tree.json endpoint for listing files in the diff

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Background

As discussed in #382236 (comment 1176331969), it helps in terms of performance when we paginate list of diff files.

Proposal

Implement diff_file_tree.json endpoint to replace the diff_files property of the diffs_metadata.json endpoint. This endpoint should support pagination params (page and per_page) just like diffs_batch.json.

A PoC has been made in !104806 (closed).