Cloud Storage URI to store all annotation results for one
client. Client should specify this field as the top-level
storage directory. Annotation results of different sessions
will be put into different sub-directories denoted by
project_name and session_id. All sub-directories will be
auto generated by program and will be made accessible to
client in response proto. URIs must be specified in the
following format: gs://bucket-id/object-idbucket-id
should be a valid Cloud Storage bucket created by client and
bucket permission shall also be configured properly.
object-id can be arbitrary string that make sense to
client. Other URI formats will return error and cause Cloud
Storage write failure.