Support loading build api credentials from GCE.

This allows using credentials to get more builds on higher-privileged
GCE instances.

Based on https://cloud.google.com/compute/docs/access/create-enable-service-accounts-for-instances

See "Authenticating applications directly with access tokens"

Bug: 137304531
Test: ./fetch_cvd
Change-Id: Ied9813f0c9264e0b6d3c77ee30c5ecd1ccd37628
8 files changed
tree: 6df8ed802b95e1c3f851343844cd14d123f4eba4
  1. common/
  2. guest/
  3. host/
  4. tests/
  5. tools/
  6. Android.bp
  7. Android.mk
  8. OWNERS
  9. TEST_MAPPING