Update the structure of the CommonMessage object

[roxanemace]

Update the common message object following @simon-temple new proto file.

syntax = "proto3";

package cm;

option java_package = "com.amalto.imdg.cm";
option java_outer_classname = "CommonMessageProtoBuff";

message CommonMessage {

    string id = 1;

    string destination = 2;
    string replyTo = 3;

    message Header {

        string key = 1;
        string value = 2;
    }

    message Attachment {

        repeated Header headers = 1;
        bytes data = 2;
    }

    repeated Header headers = 4;
    repeated Attachment attachments = 5;

}