proxy: Add support for configuring user agent

[cgwalters]

See containers/skopeo#2724

[gemini-code-assist]

Summary of Changes

Hello @cgwalters, 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 enhances the image proxy functionality by adding support for configuring a user agent prefix for skopeo commands. This allows for better identification of the client making requests, which can be useful for logging, analytics, or server-side content adaptation. The implementation includes a robust mechanism to detect skopeo's capability to handle this new option, ensuring compatibility across different skopeo versions.

Highlights

• User Agent Prefix Configuration: Introduced a new user_agent_prefix field within the ImageProxyConfig struct, allowing users to specify a custom prefix for the skopeo user agent string.
• Skopeo Feature Detection: Added a supports_user_agent_prefix() function that dynamically checks if the installed skopeo command-line tool supports the --user-agent-prefix argument by parsing its --help output. This ensures backward compatibility.
• Command Argument Integration: Modified the logic for constructing the skopeo command to conditionally include the --user-agent-prefix argument and its value, but only if the prefix is provided in the configuration and skopeo supports it.
• New Test Case: Included a new test case to validate the correct behavior of the user_agent_prefix functionality, verifying that the argument is passed when supported and omitted when not.

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[gemini-code-assist]

Code Review

This pull request introduces the ability to configure a user agent prefix for the skopeo command. It adds a new field user_agent_prefix to the ImageProxyConfig struct, a function supports_user_agent_prefix to check if the skopeo version supports the --user-agent-prefix option, and modifies the TryFrom<ImageProxyConfig> for Command implementation to include the user agent prefix if it is provided and supported. Additionally, it includes a test case to validate the new functionality. I have identified a potential issue where the supports_user_agent_prefix function might be inefficient due to repeated execution. I've provided a suggestion to memoize the result more effectively.