Add `AllIEs()` method on `Message` interface to retrieve all the IEs

[wmnsk]

It can possibly be useful if the IEs can be retrieved all at once regardless of the message type (without asserting by the message type). The example implementation for AssociationSetupRequest would be like this.

func (m *AssociationSetupRequest) AllIEs() []*ie.IE {
    ies := []*ie.IE{
        m.NodeID,
        m.RecoveryTimeStamp,
        m.UPFunctionFeatures,
        m.CPFunctionFeatures,
        m.SMFSetID,
        m.PFCPSessionRetentionInformation,
        m.PFCPASReqFlags,
    }

    ies = append(ies, m.UserPlaneIPResourceInformation...)
    ies = append(ies, m.AlternativeSMFIPAddress...)
    ies = append(ies, m.UEIPAddressPoolInformation...)
    ies = append(ies, m.GTPUPathQoSControlInformation...)
    ies = append(ies, m.ClockDriftControlInformation...)
    ies = append(ies, m.IEs...)

    return ies
}

*Maybe the inefficient append can be avoided, as the length of returned slice (=number of IEs) can be determined beforehand.

[wmnsk]

Doesn't seem to be very useful