enable the BSL service account to be used for requests

[s3than]

When GOOGLE_APPLICATION_CREDENTIALS points to a federation.json (external_account credentials), the Init() method detected the credential type but had no handling for it — falling through without setting googleAccessID. The initFromComputeEngine path (which reads serviceAccount from BSL config) was only reachable when no credentials file was present.

This adds external_account as an explicit case in the credential type switch, calling initFromComputeEngine to read serviceAccount from BSL config and set up IAM-based signing. This unblocks DownloadRequest operations (e.g. velero describe --details) for clusters using the federation webhook.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 28.02%. Comparing base (d7c2024) to head (534063b).

Files with missing lines	Patch %	Lines
velero-plugin-for-gcp/object_store.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #252      +/-   ##
==========================================
+ Coverage   27.53%   28.02%   +0.48%     
==========================================
  Files           4        4              
  Lines         443      446       +3     
==========================================
+ Hits          122      125       +3     
  Misses        309      309              
  Partials       12       12              

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