chore(rm): Automate RealmManagement API docs

- Add OpenApiSpex dependency for API documentation
- Add API annotations for RealmManagement API so API documentation
can be autogenerated and exported to yaml using

Signed-off-by: nedimtokic <nedim.tokic@secomind.com>

Author: nedimtokic

apps/astarte_realm_management/.formatter.exs

@@ -1,4 +1,4 @@
 [
-  import_deps: [:ecto, :ecto_sql, :phoenix, :skogsra, :astarte_generators],
+  import_deps: [:ecto, :ecto_sql, :phoenix, :skogsra, :astarte_generators, :open_api_spex],
   inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
 ]