Support for flag --json to apollo service:list

#### Sugestion

Would be awesome if we can pass a flag --json to `apollo service:list` to consume the output or have a programmatic version of `apollo service:list` to create nodejs scripts based on the output of this command.

#### Why

I want to create a variant schema based on another variant, to enable things like app reviews. The idea is to enable into PRs to publish an app review of a specific service and compose this service with the current staging schema.

Start by locating the implementation and output path for the `apollo service:list` CLI command, then run it against a project to understand its current output. Add a `--json` or equivalent programmatic output mode that can be consumed by Node.js scripts, and verify that the resulting service data supports the proposed schema-variant workflow.

Written by the indexing model from the issue text.