[java-storage] Improve the example of setClientEndpoint adding how to change oauth endpoint

Currently the code here [1] is being used as a reference here [2] however in an on-prem environment using Private Service Connect is also necessary to change the oauth2 token endpoint in order to get the access token.

**Is your feature request related to a problem? Please describe.**
setClientEndpoint example is not complete if you would like to use this library on a PSC environment

**Describe the solution you'd like**
Improve the current setClientEndpoint example [1]

**Describe alternatives you've considered**
Add a new section on [2], it looks like every library might need this feature as well

[1] https://github.com/googleapis/java-storage/blob/main/samples/snippets/src/main/java/com/example/storage/bucket/SetClientEndpoint.java
[2] https://cloud.google.com/storage/docs/request-endpoints

Inizia da samples/snippets/src/main/java/com/example/storage/bucket/SetClientEndpoint.java e confrontalo con la documentazione di request-endpoints. Verifica in che modo una configurazione Private Service Connect modifica l’endpoint del token OAuth2, quindi aggiorna l’esempio oppure documenta le istruzioni in modo che gli utenti possano ottenere un token di accesso in quell’ambiente.

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