Extend dynsec_clients__process_get to return which client was not found

[RoyBellingan]

Currently a request like

mosquitto_pub -t '$CONTROL/dynamic-security/v1' -u "x" -P "y" -m '
{
    "commands": [
        {
            "command": "getClient",
            "username": "PeterPan"
        }
    ]
}'

Returns

{
    "responses": [
        {
            "command": "getClient",
            "error": "Client not found"
        }
    ]
}

Because is not possible to connect which request lead to which response, I suggest to change the line

mosquitto/plugins/dynamic-security/clients.c

Line 983 in 849e0f5

dynsec__command_reply(j_responses, context, "getClient", "Client not found", correlation_data);

to contain also the username, in a somewhat similar way to what happens just a bit down below in case of a success full answer

If is ok, I can prepare the code and the PR