refactor(bigquery): return model ArrowSerializationOptions in toPb

Author: jinseopkim0

java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/ArrowSerializationOptions.java

@@ -73,7 +73,7 @@ public int hashCode() {
     return Objects.hash(bufferCompression, picosTimestampPrecision);
   }
 
-  Object toPb() {
+  com.google.api.services.bigquery.model.ArrowSerializationOptions toPb() {
     return ArrowSerializationOptionsConverter.toPb(this);
   }