added request_id to message for client testing

[arkanoider]

@grunch @bilthon ,

added an u64 named request_id in messages of mostro-core to evaluate the delay noticed in client interfaces.

pub struct MessageKind {
    /// Message version
    pub version: u8,
    /// Request_id for test on client
    pub request_id: u64,
    /// Message id is not mandatory
    #[serde(skip_serializing_if = "Option::is_none")]
    pub id: Option<Uuid>,
    /// Action to be taken
    pub action: Action,
    /// Message content
    pub content: Option<Content>,
}

[grunch]

LGTM!

[arkanoider]

uhhh mreged in main? We will keep this in the main mostro trunk?