feat: Add 'downloads add-url' command for custom tarballs

[renecannao]

Summary

Add dbdeployer downloads add-url <url> to register custom tarball URLs in the local registry.

Motivation

Users with internal mirrors, custom builds, or non-standard tarball locations need a way to add URLs to the download registry without editing JSON files manually. This is especially useful for:

• Corporate environments with internal MySQL builds
• Custom Percona/MariaDB builds
• Testing pre-release versions

Requirements

• New command: dbdeployer downloads add-url <url>
• Auto-detect version, OS, architecture, and flavor from the filename
• Add the entry to the local tarball registry
• Persist across sessions (update the local config, not the embedded registry)

Implementation Notes

• Downloads system already supports merging registries
• URL parsing logic to extract metadata from filename patterns
• Should validate URL accessibility (HEAD request) before adding