cli: add download command for benchmark datasets

## Description Add a `trackers download` CLI subcommand that lets users download benchmark datasets from a GCP bucket. Supports downloading full datasets or individual splits, with or without video frames. Datasets: MOT17, SportsMOT, SoccerNet Tracking, DenseTrack. ## Proposed CLI interface ```bash # List what's available trackers download --list # Download full dataset trackers download mot17 # Download specific split, annotations only trackers download mot17 --split train --annotations-only # Custom output path trackers download sportsmot --split val -o ./datasets ``` ## Tasks - [ ] Research and document the structure of each dataset (MOT17, SportsMOT, SoccerNet Tracking, DenseTrack): available splits, contents per split (frames, GT annotations, detections), directory layout. - [ ] Prepare zip files for each dataset/split/variant combination and upload them to a GCP bucket. - [ ] Create a dataset manifest (mapping dataset + split + variant to GCP URLs and metadata) inside the codebase. - [ ] Register the `download` subcommand in `trackers/scripts/__main__.py`. - [ ] Implement the `--list` flag to display available datasets, splits, and content types in a formatted table. - [ ] Implement download logic: resolve URL from manifest, download zip with progress bar, extract to output directory. - [ ] Handle edge cases: existing files (skip/overwrite), network errors, interrupted downloads. - [ ] Test the command end-to-end for each dataset and variant.