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.

Inizia leggendo l’entry point sagemaker.image_uris.retrieve(...) e confronta le combinazioni supportate di framework, versione del framework e versione di Python con deep-learning-containers/available_images.md. Definisci come un’API senza parametri nello stile di show_images() dovrebbe rappresentare tutte le combinazioni disponibili e valuta come verificarne la completezza rispetto ai container elencati senza richiedere ripetuti tentativi di recupero falliti.

Scritto dal modello di indicizzazione a partire dal testo della issue.