Remove OpenStruct from CLI::Args

[dduugg]

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Resolves #18777

• Retains the @table attr that OpenStruct used, but it is now exclusively for args (not metadata like named and remaining). I'm open to renaming the attr.
• missing_method dispatch is removed, only defined methods are allowed
• A generic set_arg method is provided, to allow setting args from the parser. There is no corresponding getter, because I want to encourage explicit invocations. The usual ruby escape hatches (e.g. instance_variable_get) are available if needed 😞.
• Within the args class, I've replaced arg lookup methods with @table lookups, because the methods aren't necessarily defined. It's a bit of an eyesore, but IMO preferable to defining them unnecessarily, or using respond_to? guards.

[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 defining and setting attributes instead, then we'll see what the next blocker is.

[dduugg]

Marking as ready for review. Hopefully this is a reasonable, incremental improvement over the previous code.

[MikeMcQuaid]

Thanks @dduugg! Looks good to me to merge when you're around for a few hours to deal with any post-merge 💥