Sparkle: Surface more Item values

[samford]

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Historically, the Sparkle strategy's Item struct has only included basic values from the appcast that are commonly useful. Over time we've selectively added/surfaced more values as we've encountered outliers that require use of different values in a strategy block (e.g., #13368).

We now need to use minimumSystemValue (e.g., Homebrew/homebrew-cask#158044), so this expands the Item struct to include any appcast value that we could conceivably want to use in the future. This will hopefully save us from having to make more modifications to the struct (and related tests) before we can use a previously-unused value in a strategy block.

I did livecheck runs with/without these changes and the [JSON] output remains the same, as expected.

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I will be creating a follow-up PR to reimplement the #audit_livecheck_min_os cask audit (to align with the Sparkle strategy's behavior) but we also need to be able to access item.minimum_system_version there as well. I have that work finished and will create a PR for it if/when this one is merged.

[MikeMcQuaid]

Makes sense to me, thanks @samford!