Use service identity to generate P4SA in samples and tests for Vertex AI resources

[jingyih]

Avoid hard-coding P4SA service account in Vertex AI samples and tests.

TESTED

$ go test -v -tags=integration ./config/tests/samples/create/ -run-tests vertexai-dataset-encryptionkey

$ go test -v -tags=integration ./config/tests/samples/create/ -run-tests vertexai-endpoint-encryptionkey

$ go test -v -tags=integration ./pkg/controller/dynamic/ -test.run TestCreateNoChangeUpdateDelete -run-tests vertexaidatasetencryptionkey

$ go test -v -tags=integration ./pkg/controller/dynamic/ -test.run TestCreateNoChangeUpdateDelete -run-tests vertexaiendpointencryptionkey

[maqiuyujoyce]

/lgtm
/approve

Thank you for the quick fix!

[google-oss-prow]

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This pull-request has been approved by: maqiuyujoyce

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