shareableUrl property

A link to the shared Google Photos album.

Anyone with the link can view the contents of the album, so it should be treated with care. The shareableUrl parameter is only returned if the album has link sharing turned on. If a user is already joined to an album that isn't link-shared, they can use the album's [productUrl](https://developers.google.com/photos/library/reference/rest/v1/albums#Album) to access it instead. A shareableUrl is invalidated if the owner turns off link sharing in the Google Photos app, or if the album is unshared.