feat: add BYOSA field to tuning_job

PiperOrigin-RevId: 695458857

google/cloud/aiplatform/v1beta1/tuning_job.proto

@@ -148,6 +148,15 @@ message TuningJob {
   // then all resources created by the TuningJob will be encrypted with the
   // provided encryption key.
   EncryptionSpec encryption_spec = 16;
+
+  // The service account that the tuningJob workload runs as.
+  // If not specified, the Vertex AI Secure Fine-Tuned Service Agent in the
+  // project will be used. See
+  // https://cloud.google.com/iam/docs/service-agents#vertex-ai-secure-fine-tuning-service-agent
+  //
+  // Users starting the pipeline must have the `iam.serviceAccounts.actAs`
+  // permission on this service account.
+  string service_account = 22;
 }
 
 // The Model Registry Model and Online Prediction Endpoint assiociated with