[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