Refactor code for ConfigSubCommands

[anmenaga]

Prerequisites

• [X] Write a descriptive title.
• [X] Make sure you are able to repro it on the latest version
• [X] Search the existing issues.

Steps to reproduce

3 subcomands ConfigSubCommand::Get/Set/Test use copies of mostly the same code; this should be refactored to reuse same piece of code. This was brought up in PR review #82 .

Expected behavior

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Actual behavior

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Error details

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Environment data

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Version

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Visuals

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[SteveL-MSFT]

I think a viable way to resolve this in general beyond just the 3 cases is for any struct that is intended to be rendered as JSON, we define a to_json trait that is implemented by each struct.