Support passing --insecure-policy

[pkpbynum]

This PR supports build environments that cannot supply a policy at the standard file paths (specifically nix). Eventually, I'd like this to also be consumed in composefs-rs such that we can produce composefs images at build time in the nix sandbox. In this environment, the hash of the build output is known & trusted before the image is fetched.

[gemini-code-assist]

Code Review

This pull request introduces support for the --insecure-policy flag in skopeo, allowing users to disable signature verification. The changes include adding an insecure_policy field to the ImageProxyConfig struct, implementing the logic to pass this flag to the skopeo command, and adding a corresponding test case to ensure correct functionality. The implementation is clean and follows idiomatic Rust practices, particularly with the use of Option<bool> and unwrap_or_default() for handling the boolean flag. The new feature is well-integrated and tested.