Programmatic way to get available training/inference containers.

**Describe the feature you'd like**
It would be very helpful to be able to programmatically get all available framerworks, py versions etc, prior to `sagemaker.image_uris.retrieve(...)`.
This would allow people to see what containers are available, without having to dig through repositories and try to find supported images or continuously hit errors until the right permutation of framework, framework version, and py version are achieved.

**How would this feature be used? Please describe.**
Something like `sagemaker.image_uris.show_images()` or something like that would be desired. This ideally would take no parameters and show maybe a dictionary of all the frameworks, framework versions, and respective py verisons.

**Describe alternatives you've considered**
Considering programmatically parsing: https://github.com/aws/deep-learning-containers/blob/master/available_images.md

But this does not feel like a good solution at all.

まず sagemaker.image_uris.retrieve(...) のエントリーポイントを読み、サポートされている framework、framework-version、Python-version の組み合わせを deep-learning-containers/available_images.md と比較します。パラメーターなしの show_images() スタイルの API が利用可能なすべての組み合わせをどのように表現すべきかを定義し、失敗する retrieval を繰り返し実行せずに、一覧に記載されたコンテナに対する網羅性を検証する方法を検討します。

索引モデルが issue の本文から書いたものです。