Overhaul stats events: merge UDP server events with a different IP version

[josecelano]

Change events in torrust_udp_tracker_server::statistics::event::Event from this:

pub enum Event {
    UdpRequestAborted,
    UdpRequestBanned,

    // UDP4
    Udp4IncomingRequest,
    Udp4Request {
        kind: UdpResponseKind,
    },
    Udp4Response {
        kind: UdpResponseKind,
        req_processing_time: Duration,
    },
    Udp4Error,

    // UDP6
    Udp6IncomingRequest,
    Udp6Request {
        kind: UdpResponseKind,
    },
    Udp6Response {
        kind: UdpResponseKind,
        req_processing_time: Duration,
    },
    Udp6Error,
}

To this:

pub enum Event {
    UdpRequestAborted { context: ConnectionContext },
    UdpRequestBanned { context: ConnectionContext },
    UdpIncomingRequest { context: ConnectionContext },
    UdpRequest {
        context: ConnectionContext,
        kind: UdpResponseKind,
    },
    UdpResponse {
        context: ConnectionContext,
        kind: UdpResponseKind,
        req_processing_time: Duration,
    },
    UdpError { context: ConnectionContext },
}

pub struct ConnectionContext {
    client_socket_addr: SocketAddr,
    server_socket_addr: SocketAddr,
}