Allow access to GCS client externally

[bjester]

Summary

• I missed access to the client where it was accessing the client property, which isn't available on the new Composite class added in Allow read of production Studio bucket #4926

References

Round 3: #4926

Reviewer guidance

Can you upload files after merge?

[sentry]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: contentcuration/contentcuration/utils/storage_common.py

Function	Unhandled Issue
get_presigned_upload_url	AttributeError: 'CompositeGCS' object has no attribute 'client' /api/... Event Count: 14

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