Add support to RustBoard (--load_fast) for GKE+GCS buckets.

[bmd3k]

Forked from #4784, some users using RustBoard are encountering errors when using the combination of GKE and GCS buckets.

Specifically:
#4784 (comment)
#4784 (comment)

[bmd3k]

If he's still willing, we will help @Corwinpro with his contribution.

[Corwinpro]

Thank you very much @bmd3k!

I just confirmed that the CLA is alright, so I will share something soon. FYI it will be similar to my work here https://github.com/Corwinpro/tensorboard/pull/1/files.