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.

Empieza leyendo el punto de entrada sagemaker.image_uris.retrieve(...) y compara sus combinaciones compatibles de framework, versión del framework y versión de Python con deep-learning-containers/available_images.md. Define cómo debería representar todas las combinaciones disponibles una API sin parámetros al estilo de show_images(), y considera cómo se puede verificar la integridad con respecto a los contenedores enumerados sin requerir intentos repetidos de recuperación fallidos.

Escrito por el modelo de indexación a partir del texto del issue.