Fix json.rb 2.7.2 compatibility

[factyy]

Stdlib's json.rb now (since version 2.7.2) relies on GenericObject which is autoloaded later (see https://github.com/ruby/ruby/blob/9d69619623ec6b86c464b7cac911b7201f74dab7/ext/json/lib/json/common.rb#L5).

This change makes Graphiti unable to be required.

If you try to require 'graphiti' you end up with the following error:

...gems/graphiti-1.7.2/lib/graphiti/resource/remote.rb:8:in `block in <module:Remote>': uninitialized constant Graphiti::Resource::Remote::OpenStruct (NameError)

        self.model = OpenStruct
                     ^^^^^^^^^^
	from ...gems/activesupport-7.1.3.4/lib/active_support/concern.rb:138:in `class_eval'
	from ...gems/activesupport-7.1.3.4/lib/active_support/concern.rb:138:in `append_features'
	from ...gems/graphiti-1.7.2/lib/graphiti/resource/configuration.rb:48:in `include'
	from ...gems/graphiti-1.7.2/lib/graphiti/resource/configuration.rb:48:in `remote='

This PR simply requires OpenStruct explicitly (if not already loaded).

[jkeen]

Thanks!

[github-actions]

🎉 This PR is included in version 1.7.3 🎉

The release is available on:

• v1.7.3
• GitHub release

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