Add download helper utilities with integrity verification, metadata tracking, and anti-blocking features

[jeffryhawchab]

Summary

Enhances apkeep with download helper utilities including metadata tracking, SHA256 integrity verification, and customizable anti-blocking measures for more reliable and secure APK downloads from all supported sources (APKPure, F-Droid, Google Play, Huawei AppGallery).

New Features

Download Helper Utilities (src/util/download_helper.rs)

• DownloadMetadata struct: Track comprehensive download information (app ID, version, filename, SHA256, file size, URL, timestamp, source)
• SHA256 checksum computation: Automatic integrity verification after download
• Metadata persistence: Save/load download metadata as JSON files alongside APKs
• Resume support: Detect and resume interrupted downloads
• Anti-blocking headers: Customizable User-Agent and HTTP headers with browser-like defaults

New CLI Options

• --user-agent <agent>: Custom User-Agent string (default: Chrome/Windows)
• --headers <headers>: Custom HTTP headers (format: 'Header1:Value1,Header2:Value2')
• --timeout <seconds>: Request timeout (default: 300)
• --verify: Verify APK integrity using SHA256 checksum
• --save-metadata: Save download metadata as JSON
• --skip-existing: Skip existing files instead of resuming

Integration

• Integrated helper functions into APKPure download source
• Helper functions available for all download sources (APKPure, F-Droid, Google Play, Huawei AppGallery)
• CLI options passed through to download sources via options map

Bug Fixes

• F-Droid extraction: Improved error handling in ZIP extraction to prevent panics on corrupted archives

Documentation & Testing

• Updated README with new features and usage examples
• Updated USAGE file with new CLI options
• Added comprehensive unit tests for all helper functions
• Enhanced .gitignore to exclude build artifacts

Usage Examples

# Download with integrity verification
apkeep -a com.instagram.android --verify .

# Download with custom anti-blocking headers
apkeep -a com.instagram.android --user-agent "CustomBot/1.0" --headers "Accept:application/json" .

# Download with metadata tracking
apkeep -a com.instagram.android --save-metadata .

# Works with all sources
apkeep -a org.mozilla.fennec_fdroid -d f-droid --verify --save-metadata .