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.