BigQueryStorage `PERMISSION_DENIED` in the `WriteCommitedStream` sample

[bphenriques]

Mirroring issue #196338290.

The sample WriteCommitedStream is passing the client onto JsonStreamWritter leading to PERMISSION_DENIED even if all the pre-conditions for using the API are met.

Let me know if I need add more details on the issue. I omitted as they are described in the issue tracker.

BigQuery Storage version: 2.0.2