Add user-agent string for Beam in the GCS request header.

[shunping]

The user-agent string was added into gcsio in PR 27211. However, due to the fact that google-cloud-storage<2.13.0 does not support extra header, we did not keep user-agent during the migration PR 29360.

Now, as we upgrade google-cloud-storage dependency to the latest version (2.14.0), we can support user-agent string in the request header again.

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