Custom details on payload

# Description
Can we add in custom fields with custom information to allow for more details on the message.

## What
MessageFactory. has a map called payload, can we do something this?

Map payload = new HashMap();
payload.put("summary", modelData.event().message());
payload.put("source", "Graylog:" + modelData.event().sourceStreams());
payload.put("severity", eventPriority);
payload.put("timestamp", modelData.event().eventTimestamp().toString());
payload.put("component", "GraylogAlerts");
payload.put("group", modelData.event().sourceStreams().toString());
payload.put("class", "alerts");
payload.put("details", customDetailsMap.toString());

## Why
This will help with event alerting