feat: Add list passwords for MacOS Keychain

[dacamposol]

Context

This pull request introduces a new feature to the macOS backend of the keyring library, enabling users to list all generic passwords stored in the keychain.

Key Changes

New Method Added

1. list_passwords method is added to macOS.Keyring, allowing retrieval of all generic passwords.
2. This method uses the new api.list_generic_passwords() function.

API Enhancements

Implements list_generic_passwords() in macOS/api.py, which:

1. Queries the keychain for all generic password items.
2. Returns a list of dictionaries containing the service and account for each item.
3. Adds helpers for converting CoreFoundation types (CFStringRef, CFDataRef) to Python strings for easier handling of keychain data.

Testing

A new test test_list_generic_passwords in tests/backends/test_macOS.py verifies:

1. The password listing functionality.
2. That passwords added by the test can be successfully found and then cleaned up.

Impact

• User Benefit: macOS users can now programmatically list all generic passwords in their keychain using keyring’s API.
• Testing: The feature is covered by new automated tests.
• Backward Compatibility: Changes are additive and should not affect existing functionality.

TLDR; This PR enhances the macOS backend by adding password listing capability, improves type conversions, and provides robust tests for the new functionality.

[dacamposol]

@jaraco or @mitya57, could you take a look?

[mitya57]

I don't use macOS, and don't have experience with its API, so I leave the review to @jaraco.

One thing I would ask, though, is to not mix functional changes and style changes (like replacing quotes) in the same commit.