Add `Gitlab::Utils::HashStruct` class

What does this MR do and why?

In the scope of #344279, we've started actively to use ActiveSupport::InheritableOptions. See !75280 (comment 843625918).

Later, we noticed that ActiveSupport::InheritableOptions doesn't build an instance from a hash with stringified keys.

This MR adds Gitlab::Utils::StructHash to address this.

Related to !75280 (comment 847633646)

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