`ProcessingS3Output`'s `s3_uri` to be an optional field

**Describe the feature you'd like**
In the SDK v2, `ProcessingOutput` has a `destination` parameter to be either an S3 path where users want to write their results, but they can set it to `None` when they want to delegate the storage to SageMaker. This option stores the results in an S3 prefix partitioned by `job_name`, `step_name`, and `output_name`, which is quite convenient for traceability and to store intermediate results of a pipeline.

**How would this feature be used? Please describe.**
Much like v2, v3 should also support this feature. Users may not want to fix the destination path for intermediate results, so ideally the same behaviour should apply to v3 by leaving `ProcessingS3Output`'s `s3_uri` field as `None`. Currently this field is mandatory and it's unclear whether users can reproduce the v2 `None` behaviour.

**Describe alternatives you've considered**
No alternatives considered so far other than sticking to v2.

**Additional context**
N/A

Inizia confrontando il comportamento di SDK v2 ProcessingOutput con la definizione di v3 ProcessingS3Output e la relativa gestione dei valori di destinazione. Conferma come debba essere rappresentato un S3 URI omesso per gli output intermedi; il lavoro è completato quando v3 supporta il caso d’uso documentato equivalente a v2 e verifica il comportamento con test appropriati.

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