Add support for keyboard-interactie auth.

[mateuszkj]

What

This change adds support for the keyboard-interactive authentication method described in RFC 4256.

Implementation

This solution introduces the AuthKeyboardInteractive builder, which can be converted into an AuthMethod.

let kbi = AuthKeyboardInteractive::new()
    .with_response("Password: ", "abt")
    .with_response("OTP: ", "12345");

let auth = AuthMethod::with_keyboard_interactive(kbi);
...

This implementation allows defining responses to known and exact prompts.

Implementation Considerations

Although my solution addresses my problem, it might be too limited for others. Here are some considerations:

• Should prompts be trimmed of white spaces before comparison?
• Should prompt comparisons be case-insensitive?
• Should we add an async handler so API users can implement custom responses (e.g., asking SMS services for OTPs)?

If the answer to any of these questions is yes, I am willing to implement those changes.

[mateuszkj]

@Miyoshi-Ryota Thanks for merging it.

If you need help with any implementation, feel free to contact me. I'm also considering adding SSH-agent support, so there might be a need for an async credential provider then.