Add an MS MARCO dataset importer

[AshwinUgale]

The problem / use case

Context: cli.py has import-squad and import-beir that emit docs.jsonl +
queries.jsonl with source-span gold. MS MARCO is a natural third target.

What: Add an import-msmarco subcommand that converts an MS MARCO passage/qrels
export into the same format.

Acceptance:

• retrieval-lab import-msmarco … --out-dir … produces valid docs.jsonl + queries.jsonl
• gold is documented as passage-level (coarser/stricter than answer-span), matching how
  BEIR import already frames it
• a small fixture test in tests/test_datasets.py; CHANGELOG.md updated

Pointers: mirror import-beir (passage-level relevance → span over the whole passage).

Proposed idea

Add import-msmarco alongside the SQuAD/BEIR importers so users can benchmark on a well-known passage-ranking dataset; its passage-level relevance imports as coarser (passage-span) gold, stated honestly.