Add blank published toolchain configs json file (#287)

Will be used to store the list of toolchain configs there were published
to this repository and will be automatically updated by the configs
publisher flow.
diff --git a/configs/published_toolchain_configs.json b/configs/published_toolchain_configs.json
new file mode 100644
index 0000000..7dd4387
--- /dev/null
+++ b/configs/published_toolchain_configs.json
@@ -0,0 +1,2 @@
+[]
+