Allow configuration of cli settings via a JSON file

[ellismg]

Today, we have a collection of environment variables and XML files that provide configuration to the toolchain. For example, we have an environment variable to to control if telemetry is reported. We have another that you can use to change where the machine wide nuget configuration is read from (so folks can add additional sources).

It would be nice if this configuration could be centralized in some sort of config.json file SxS with the SDK files, to make it easier for folks to control instead of having to add tons of environment variables to their command lines or environment.

[TheRealPiotrP]

@ellismg do we have an explicit list of the configuration you care about? If this is going to make preview2 it needs to come in fairly soon. You should also note that the env vars you mention, NuGet reads some and CLI reads others...

[ellismg]

Can we start with the environment variables that CLI reads (I know the telemetry one, I was unaware there were others).

Ignoring the NuGet ones for preview fine is reasonable, we can figure out the best way to centralize them later.

[blackdwarf]

Generally agree that this would be a good overall usability addition, so moving to RTM for further scoping and proper evaluation.