Support using service account credentials in acloud fetch_cvd

Bug: 228338591
Test: acloud-dev create --branch git_master --build-target
aosp_cf_arm64_only_phone_hwasan-userdebug  --build-id 8584085
--remote-fetch --host fl1-arm-06.atc.google.com --host-user
android-test-admin --service-account-json-private-key-path
/usr/local/google/home/yuchenhe/key.json -vv; unit test

Change-Id: I60e4b6e5e214adc80381dbfe3e227c7f8cc6e85a
2 files changed