[ingress] /restate/invocation/:invocation_id/status to retrieve invocation status - #5178

#[derive(Debug, Serialize)]
#[serde(rename_all = "lowercase")]
pub enum InvocationStage {
    /// Means the invocation has been created but not yet started.
    Created,
    /// The invocation started and has not yet completed.
    Started,
    /// The invocation completed, with either success or failure.
    Completed,
}

#[derive(Debug, Serialize)]
pub struct InvocationStatusResponse {
    stage: InvocationStage,
    /// Filled if `stage = 'completed'` and the invocation failed
    error: Option<InvocationError>,
}