Add artifacts_download module and example for universal packages

[cataggar]

Summary

Fixed #330. Adds an artifacts_download module and example for downloading universal packages from Azure DevOps Artifacts. The module provides a Client/ClientBuilder using azure_core for HTTP, following the same patterns as other modules in the crate.

Download Protocol

The module implements a 5-step dedup-based download protocol:

1. Service discovery — GET /{org}/_apis/ResourceAreas to find the packages and blob service URLs
2. Package metadata — GET {pkgsHost}/{project}/_packaging/{feed}/upack/packages/{name}/versions/{version} to get manifest and super-root IDs
3. Blob URL resolution — POST {blobHost}/_apis/dedup/urls to convert blob IDs into SAS-signed download URLs
4. Manifest download — Fetch and parse the JSON manifest listing all files and their blob references
5. Chunk download & decompression — For each file, resolve its dedup node, download content chunks, decompress, and concatenate

Usage

export ADO_TOKEN=
export ADO_ORGANIZATION=myorg
export ADO_PROJECT=myproject

cargo run --example download_artifact --features=artifacts_download -- \
  --feed my-feed \
  --name my-package \
  --version 1.0.0 \
  --path /tmp/output

Verification

Tested end-to-end against a real Azure DevOps feed. The downloaded file matches byte-for-byte against the output of az artifacts universal download.