livecheck: update types to allow nested JSON hashes

[khipp]

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This PR allows the JSON data to use when making a POST request to be nested.

Unblocks: Homebrew/homebrew-cask#205110

[MikeMcQuaid]

Looks ok but if either of the suggested versions can work instead that'd be preferable ❤️ Thanks!

[p-linnane]

Thanks @khipp!