Allow setting DAG outputs without the SDK/Client

[polyaxon-team]

Current behavior

Currently users who need to set outputs on a DAG need to use the SDK/Client to push an outputs from an operation to the parent pipeline. A recurrent question is similar to this slack thread

is there a similar way to make one of the outputs of ops in a dag as the output of the dag?

outputs:
  - name: dag_output
    ref: ops.dag_operation_1
    value: outputs.dag_operation_1_output_name

Enhancement

Allow users to pass outputs/results to the pipeline directly from the Polyaxonfile specification.