core: support aria2c as an optional downloader for platform/library installs (like Scoop)

[fedcba499]

Describe the request

Request: optionally use aria2c for downloads, similar to how Scoop does it on Windows — parallel, multi-connection downloads for much faster installs. Fall back to the current downloader if aria2c isn't installed.

Describe the current behavior

Currently arduino-cli downloads platform/tool files one at a time. For large platforms like esp32 (15+ files), this is slow.

Arduino CLI version

1.5.1

Operating system

Windows

Operating system version

Windows 10 22H2

Additional context

No response

Issue checklist

• I searched for previous requests in the issue tracker
• I verified the feature was still missing when using the nightly build
• My request contains all necessary details