Feat/registries auth mecanism

[Leay15]

This pull request rebases on #351, it introduces robust registry authentication support for image pulling and pushing in the microsandbox CLI and core library.
It adds:

• environment variable and persistent credential management
• new CLI commands for login/logout
• a unified resolution logic for registry authentication.

These improvements make private registry access easier, safer, and more consistent for users and developers.

Registry authentication and credential management

• Added new CLI commands: msb login and msb logout for storing and removing registry credentials locally, with detailed documentation and security notes in docs/references/cli.md.
• Introduced environment variable support for registry credentials (MSB_REGISTRY_TOKEN, MSB_REGISTRY_USERNAME, MSB_REGISTRY_PASSWORD, etc.), which take priority over stored credentials. [1] [2]
• Credentials are now stored in ~/.microsandbox/registry_auth.json with guidance to restrict file permissions.

Core authentication resolution logic

• Implemented resolve_registry_auth, normalize_registry_host, and registry_host_for_reference functions in microsandbox-core/lib/oci/auth.rs, providing unified logic for resolving registry authentication based on environment, stored credentials, and fallback to anonymous access.
• Added comprehensive unit tests for authentication resolution, including precedence and error handling. [1] [2]

Integration into image pull/push workflow

• Updated Registry::new and image pull logic to accept and use resolved registry authentication, ensuring all image operations use correct credentials. [1] [2] [3] [4] [5]
• Exposed registry auth resolution via compatibility re-exports for management code. [1] [2] [3] [4]

CLI argument and option enhancements

• Extended MicrosandboxSubcommand enum to support login and logout commands with registry, username, password, token, and --all options.

Utility and dependency updates

• Added new dependencies (serde, serde_json, base64) for credential serialization and management in microsandbox-utils.

These changes collectively enable secure, flexible, and user-friendly registry authentication for microsandbox image workflows.

[Leay15]

This PR was open by request on #364