chore: Set up CODEOWNERS for openapi specs (#1495)

- this is so that @netmilk and somebody from Tooling team (I volunteer
as tribute) know when something changes in the OpenAPI spec
- we should also add somebody from the platform team - @fnesveda, what
do you think?

Author: janbuchar

.github/CODEOWNERS

@@ -6,3 +6,6 @@
 
 # Academy
 /sources/academy/ @honzajavorek
+
+# OpenAPI spec
+/apify-api/ @netmilk @janbuchar @fnesveda