[core] Retrieve the token from GCS server [4/n] (#37003)

[vitsai]

Retrieve the token from the GCS server in the GCS client while connecting, to attach to metadata in requests.

Previous PR (GCS server): #36535
Next PR (auth): #36073

Why are these changes needed?

Related issue number

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    method in Tune, I've added it in doc/source/tune/api/ under the
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• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[jjyao]

cc @zhe-thoughts for approval.

[zhe-thoughts]

Let's pick since this is a release blocking issue