feat(firebase_ai): add LiveServerGoAway message for session termination

[SelaseKay]

Related Issues

Fixes #17836

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[cynthiajoan]

This change would require API review within the team. I started to looking into the feature now.

[cynthiajoan]

with the API review, let's update the name to the follow

/// A server message indicating that the server will not be able to service the
/// client soon.
class GoingAwayNotice implements LiveServerMessage {
/// Creates a [GoingAwayNotice] instance.
///
/// [timeLeft] (optional): The remaining time before the connection will be
/// terminated.
GoingAwayNotice({this.timeLeft});

/// The remaining time before the connection will be terminated as ABORTED.
final Duration timeLeft;
}

Besides that, PR lgtm