Expose operations_client

[busunkim96]

To use the operations client that you have to go through client._transport. It feels weird to recommend using a variable that looks like it's intended for internal use only.

Published AutoML sample: https://github.com/googleapis/python-automl/blob/master/samples/snippets/get_operation_status.py

def get_operation_status(operation_full_id):
    """Get operation status."""
    # [START automl_get_operation_status]
    from google.cloud import automl

    # TODO(developer): Uncomment and set the following variables
    # operation_full_id = \
    #     "projects/[projectId]/locations/us-central1/operations/[operationId]"

    client = automl.AutoMlClient()
    # Get the latest state of a long-running operation.
    response = client._transport.operations_client.get_operation(
        operation_full_id
    )

    print("Name: {}".format(response.name))
    print("Operation details:")
    print(response)
    # [END automl_get_operation_status]

It looks like the old generator lefttransport public. Was that changed intentionally? https://github.com/googleapis/gapic-generator/blob/17fbb3b38a20d13e69879d845a045dc1f8448a3d/src/test/java/com/google/api/codegen/gapic/testdata/py/python_multiple_services.baseline#L999

If we want to keep transport private, could we make operations_client public somewhere else?