Support configurable concurrent package downloads during bulk upgrades #6595

Relevant area(s)

WinGet CLI

Description of the new feature / enhancement

Please support downloading multiple package installers concurrently during winget upgrade --all, while keeping installation sequential. The current download-install-download-install flow leaves the network idle during installation.

Homebrew provides a useful example with HOMEBREW_DOWNLOAD_CONCURRENCY: https://docs.brew.sh/Manpage#environment

Proposed technical implementation details

• Use a bounded download queue with configurable concurrency.
• Continue downloads while one installer runs; respect dependency ordering.
• Allow concurrency 1 for serial behavior.
• Show per-package progress and retain integrity checks.

Related: #4105 decouples download and installation; this request adds configurable concurrent downloads across multiple packages. #225 concerns multiple connections for one file. Please consolidate under #4105 if appropriate.