FR(storage-transfer): Export Required Protobufs if Required By Library

[danielbankhead]

Is your feature request related to a problem? Please describe. Required protobufs should be exported directly if required by the library. Currently, google.protobuf must be imported in order to be used - this isn't ergonomic and requires a bit of digging to determine how to retrieve the desired type.

Example: Duration is required for certain configurations, such as min_time_elapsed_since_last_modification:

from google.protobuf.duration_pb2 import Duration

transfer_job_request = storage_transfer.CreateTransferJobRequest({
    'transfer_job': {
        'project_id': project_id,
        'description': description,
        'status': storage_transfer.TransferJob.Status.ENABLED,
        'schedule': {
            'schedule_start_date': {
                'day': start_date.day,
                'month': start_date.month,
                'year': start_date.year
            }
        },
        'transfer_spec': {
            'gcs_data_source': {
                'bucket_name': source_bucket,
            },
            'gcs_data_sink': {
                'bucket_name': sink_bucket,
            },
            'object_conditions': {
                'min_time_elapsed_since_last_modification': Duration(
                    seconds=2592000  # 30 days
                )
            },
            'transfer_options': {
                'delete_objects_from_source_after_transfer': True
            }
        }
    }
})

Describe the solution you'd like

Required protobufs should be exported. It should be (or similar to):

from google.cloud.storage_transfer import Duration

Describe alternatives you've considered The only alternative is to import from google.protobuf.duration_pb2:

from google.protobuf.duration_pb2 import Duration

[cojenco]

Transferred issue to monorepo as this is a feature request to the storage-transfer python client - required protobufs should be exported directly if required by the library cc @danielbankhead

[parthea]

Moving to gapic-generator-python as this feature request is for the code generator.