Add json support to --info output

[steveharter]

During discussion of https://github.com/dotnet/core-setup/issues/3502 a feature to add json support to dotnet --info was discussed as a way for developers to easily obtain information by parsing json.

Information includes the SDK version, available SDK versions, and available runtimes. Being json, it would be extensible to other additional information in the future and thus serve as general API to return environmental information.

To enable the mode the proposal is an option on --info like this:
dotnet --info --format json
We could also add a default format as shown today:
dotnet --info --format text

Actual json syntax is TBD; marking this as future

[valdisiljuconoks]

please add support for this also for list-runtimes and list-sdks

[steveharter]

Setting to 3.0 as there have been several requests

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