Support for JSON schema codegen target

The [apollo client:codegen](https://github.com/apollographql/apollo-tooling#apollo-clientcodegen-output) command currently supports swift, typescript, flow and scala. It would be great if it could support outputting a json schema too.

We have a tool that can generate stub data which takes a json schema as an input. Right now we use the typescript output and then another tool to convert the typescript output to a json schema.

I think this would involve adding a new `apollo-codegen-json` package.

I'm happy to pick the work up on this, but was wondering if it is something that would be accepted first.

Start with the apollo client:codegen command and compare its existing swift, typescript, flow, and scala targets. Determine how a new apollo-codegen-json package would fit and verify that the command can emit a JSON Schema accepted by the referenced stub-data tool.

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