[java-storage] RemoteStorageHelper possibility to provide own Credentials

**Is your feature request related to a problem? Please describe.**
Currently, while using `RemoteStorageHelper`, you can only use `keyStream – input stream for a JSON key` as credentials for storage. I want to provide custom `com.google.auth.Credentials` because of how our internal systems manage access to Google Cloud.

**Describe the solution you'd like**
I want to add a method to `RemoteStorageHelper` that would enable providing the `com.google.auth.Credentials`.
```
public static RemoteStorageHelper create(String projectId, Credentials credentials)
```