Configure Kokoro for releases (#6733)

Author: tswast

.kokoro/release/bigquery_storage.cfg

@@ -0,0 +1,7 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Tell the trampoline which build file to use.
+env_vars: {
+    key: "PACKAGE"
+    value: "bigquery_storage"
+}