Add support for JSON output

[joshwolfer]

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Is your feature request related to a problem? Please describe.

Not related to a problem.

Feature Description

I would like to be able to take the output from certain commands and easily consume it to be re-usable. For example:

$ consul-k8s proxy list -n unicorn
Namespace: unicorn

Name                            Type
unicorn-frontend-97848474-mkczg Sidecar

I would like to have the output presented in json so I can quickly resolve the proxy list and then do additional things.

Similar to how kubectl does it:
consul-k8s proxy list -n unicorn -o json

Use Case(s)

Explained above

[david-yu]

Closed by #3221