Add getProjectNumber method to Bucket.java #3023

Is your feature request related to a problem? Please describe.

The Java SDK for GCS does not have a method for fetching the project number even though the value is returned from the describeBucket API. This helper exists on the C# SDK as per: https://cloud.google.com/storage/docs/getting-bucket-metadata#storage-get-bucket-metadata-csharp.

Please also see also JSON API reference: https://cloud.google.com/storage/docs/json_api/v1/buckets#resource

Describe the solution you'd like
Add the method for returning the projectNumber. I intend to open a PR shortly adding the required method.

Describe alternatives you've considered
n/a