Pager Duty Additional fields

# Description
When using the pager duty plugin, there is not enough information in the payload to include extra fields. This change / patch allows for extra information to be placed in the org/graylog/integrations/pagerduty/client/MessageFactory.java class at line 87 - 98

## What
ImmutableList immutableList = modelData.backlog();
StringBuilder stringBuilder = new StringBuilder();
for (Object objectSummary : immutableList) {
MessageSummary summary = (MessageSummary)objectSummary;
String message = summary.getMessage();
stringBuilder.append(message);
stringBuilder.append("\n");
}
String messageDetails = stringBuilder.toString();

payload.put("custom_details", messageDetails);

## Why

Having this custom field allows for clients to publish critical information on the alerts to better route them.