Enhancement: workload identity support for GKE deployments to GCS storage

[todpunk]

Basically, I want to use workload identity, as it is more secure than static credentials I have to rotate and less env vars to set in the first place. Without changing previous use cases configured for GCS either with RSA or the S3 access key/id method, we can just adjust backend/src/storage/gcs.rs to have a different mode. I have a PR I'll link here in a moment with my initial attempt at that.

[todpunk]

#334

[todpunk]

Per #334 superceded by #350 which resolves this