get_object_cdn_url() & get_container_cdn_url() implementation for StorageDrivers

## Feature Request

Hi all! After beginning to integrate the libcloud library into an application I am writing and attempting to use both the `get_object_cdn_url()` and `get_container_cdn_url()`, I have realized that these methods appear to not be implemented back at the base level (`libcloud/storage/base.py`) in the StorageDriver class, and never implements in any inherited classes. I am attempting to use the Google driver, which inherits from the S3 driver, which inherits from the base.py StorageDriver, and looking at these methods in there, it appears they are hardcoded to raise the `NotImplementedError`: _"get_container_cdn_url not implemented for this driver"_. You can see that [here](https://github.com/apache/libcloud/blob/trunk/libcloud/storage/base.py) on lines 399 and 430.

I just wanted to confirm that I am indeed right about this, and see if there is already a roadmap that includes this feature. If not, I may be willing to take it on depending on the implications to the rest of the codebase. Apologies if I am not understanding the codebase correctly, I don't have any experience at all with libcloud besides this quick integration.

**Does anyone have anything thoughts, reservations, ideas, on this?**

Here is a chunk of code using a Google Cloud Storage instance to replicate this not implemented error:

`> import libcloud`
`> driver = libcloud.storage.drivers.google_storage.GoogleStorageDriver(key='foo', secret='bar')`
`> driver.get_container_cdn_url('container name')`

Bắt đầu trong libcloud/storage/base.py tại get_container_cdn_url() và get_object_cdn_url(), sau đó lần theo GoogleStorageDriver qua quan hệ kế thừa S3 của nó. Xác định những driver nào có thể cung cấp URL CDN và hành vi nào của provider là bắt buộc; công việc được xem là hoàn tất khi các triển khai được hỗ trợ không còn chạm đến NotImplementedError của lớp cơ sở và có coverage cho các driver bị ảnh hưởng.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.