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')`

libcloud/storage/base.py の get_container_cdn_url() と get_object_cdn_url() から始め、GoogleStorageDriver を S3 の継承関係に沿って追跡します。どのドライバが CDN URL を提供できるか、またプロバイダにどのような動作が必要かを確認します。対応する実装が base の NotImplementedError に到達しなくなり、影響を受けるドライバがカバレッジで網羅されれば完了です。

索引モデルが issue の本文から書いたものです。