feat: cache downloaded packages

[murfffi]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @murfffi, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the package downloading functionality by introducing an intelligent caching mechanism. By utilizing HTTP ETag and Content-Length headers, the system can now avoid re-downloading packages that have not been updated, leading to improved performance and reduced network usage. Concurrently, the PR refactors the codebase to better organize download-related logic, separating concerns for different download sources and centralizing common utility functions, which results in a more maintainable and robust system.

Highlights

• Package Caching Implementation: Introduced a new caching mechanism for downloaded packages, leveraging HTTP ETag and Content-Length headers to prevent redundant downloads of unchanged files. This significantly improves efficiency by reusing previously fetched resources.
• Refactored Download Logic: The logic for handling GitHub-specific downloads, including URL generation and retrieving the latest version path, has been extracted into a new dedicated file, download/github.go, enhancing modularity and separation of concerns.
• Centralized Binary Prefix Generation: A new BinType.Prefix() method was added to centralize the generation of binary prefixes (e.g., 'duckdb_cli', 'libduckdb'), making the code more consistent and maintainable across different download functions.
• API Updates for Spec and Result: The Spec struct now includes a CacheDownload boolean option, which is enabled by default in DefaultSpec(). Additionally, the Downloaded field in the Result struct has been renamed to OutputWritten for clearer semantics.

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[gemini-code-assist]

Code Review

This pull request introduces caching for downloaded packages, which is a great enhancement to reduce network traffic and improve speed on subsequent runs. The code has been nicely refactored to separate GitHub-specific logic. However, I've found a couple of issues in the new caching implementation within fetchZip. The logic for deciding when to cache is flawed, and the ETag value is not sanitized before being used in a filename, which could lead to errors on some operating systems. My review includes a detailed comment with a code suggestion to address these points.