Builds a PubsubMessage Object
PubsubMessage(
  data = NULL,
  message_id = NULL,
  ordering_key = NULL,
  attributes = NULL,
  publish_time = NULL
)PubsubMessage object
character The message data field as a base64 encoded string
character ID of this message, assigned by the server when the message
is published
character If non-empty, identifies related messages for which publish
order should be respected
list Key-value pairs attributes for this message
character The time at which the message was published, populated by the server
when it receives the Publish call
Other Object functions: 
DeadLetterPolicy(),
DlqPolicy(),
ExpirationPolicy(),
MessageStoragePolicy(),
PushConfig(),
RetryPolicy(),
SchemaSettings(),
Schema(),
Snapshot(),
Subscription(),
Topic()