Support config get-contexts JSON output

[hclihn]

What would you like to be added:
Please add support for config get-contexts JSON output.

Why is this needed:
Currently, config get-contexts can only output column-oriented format or the name field (using -o=name). However, when the cluster/context name contains space(s) which is true for our company's implementation, it makes the parsing of the output very challenging. If the command output can support a JSON format, then it's relatively easier to use existing tools to parse it, regardless if the cluster/context name contains space(s) or not.

[ardaguclu]

Sounds to me a reasonable ask.

[ah8ad3]

I'll take a look at this.

[ardaguclu]

@ah8ad3 before working on it, I think it would be better to wait bug scrub to triage it first.

[mpuckett159]

/triage accepted

[TessaIO]

/assign