Remove usage of OpenStruct or add ostruct as dependency

[Earlopain]

Issue description

Hey there, I saw #1110 which added a require of ostruct to work around an error.

ostruct is on its way to be a bundled gem and Ruby 3.4 will warn if it is required without being specified as a dependency. ruby/ruby#10428

So I suggest removing the usage to avoid breaking in Ruby 3.5. Adding it to the gemspec is also a solution.