feat: Add by_index_with_options() for ignoring encryption

[b-spencer]

This PR adds support for completely ignoring the encrypted flag on a zip file entry. When ignored, the entry is treated as if it were plaintext.

Zip archives sometimes intentionally misrepresent their content. Some malware archives do this by marking a file entry as encrypted when it actually isn't. Since some real-world zip readers ignore the encryption flag, those applications will read, and possibly be harmed by, such archives. This feature allows malware analysis tools using this crate to read such archives.

This PR replaces the private by_index_with_optional_password() with an extensible public by_index_with_options() to allow multiple indepedent optional arguments to the operation. The new ZipReadOptions builder struct allows specifying an optional password and setting the ignore_encryption_flag option, which, of course, defaults to false.

[gemini-code-assist]

Summary of Changes

Hello @b-spencer, 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 zip archive reading capabilities by introducing a flexible options-based approach for retrieving individual entries. The primary motivation is to enable robust handling of archives that intentionally misrepresent their content, such as malware archives marking unencrypted files as encrypted. By allowing users to explicitly ignore the encryption flag, the library becomes a more powerful tool for analysis, while also providing a cleaner, more extensible API for future reading options.

Highlights

• New by_index_with_options() Method: Introduced a new public method by_index_with_options() on ZipArchive to provide a more flexible way to read individual entries from a zip archive, replacing the private by_index_with_optional_password().
• Flexible ZipReadOptions Struct: Added a new builder struct ZipReadOptions which allows specifying optional parameters like a password and a new ignore_encryption_flag when reading zip entries. This promotes extensibility for future options.
• Ignore Encryption Flag Feature: Implemented the ability to completely ignore the encrypted flag on a zip file entry. When enabled via ZipReadOptions, the entry is treated as if it were plaintext, even if the flag indicates encryption. This is particularly useful for analyzing malformed or malicious archives.
• API Refactoring: Existing public methods such as by_password(), by_path(), by_name_decrypt(), by_index_decrypt(), and by_index() have been updated to internally utilize the new by_index_with_options() method, streamlining the codebase and centralizing entry reading logic.

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[gemini-code-assist]

Code Review

This pull request introduces a valuable feature to ignore the encryption flag on zip entries, which is particularly useful for malware analysis. The implementation is solid, replacing an internal function with a more extensible public API using the ZipReadOptions builder pattern. The new logic correctly handles the ignore_encryption_flag and the accompanying tests provide good coverage for the new functionality and ensure no regressions.

I have a couple of suggestions to improve the ergonomics of the new ZipReadOptions builder API.