Remove OpenStruct from CLI::Args

[dduugg]

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Resolves #18777

Notes incoming, draft for now to check downstream effects

[dduugg]

Promising start, passes tests locally, but has an issue where methods are being defined twice – once when the cmd_args are read, and once when the args are set. I should be able to refine this approach by refining the approach to define and set attributes instead, then we'll see what the next blocker is.