Storage: Expose the get method of Notifications

[HemangChothani]

-> Expose the get method for BucketNotification class

Json API documentation: https://cloud.google.com/storage/docs/json_api/v1/notifications/get

Related Feature request in java: googleapis/java-storage#138

/cc: @frankyn @crwilcox

[HemangChothani]

Closing this Feature request as found reload method work as get notification.

[HemangChothani]

@frankyn @crwilcox Need a suggestion.
Is it ok to expose a method which get a notification from notificationId?

Method:
bucket.get_notification(notification_id)

[frankyn]

Yes, this is an issue with usability with get_notification, in which get by Id is not available.

The changes should be minimal to the existing surface by making:

bucket.notification(): make topic_name optional and surface notification_id.

notification = bucket.notification(id="notification-id")
notification.reload()

# Potentially
notification_with_metadata = bucket.get_notification(id="notification-id")

[frankyn]

Updated comment with potential examples.

Note: please prioritize v4 signature requests that haven't started:
https://github.com/googleapis/python-storage/issues?utf8=%E2%9C%93&q=is%3Aissue+is%3Aopen+v4