Provide an environment export to support Offline Mode

[zachaysan]

See this related ticket.

Now that we're supporting an increasingly larger number of SDKs operating in Offline Mode, it would be a real usability win if an environment.json file could be generated for our users which can then be manually imported into their SDKs for use in Offline Mode.

@matthewelwell has some ideas on how to make this export really straight forward.

[matthewelwell]

User token authentication (instead of Server Key authenticated) endpoint already exists for this at GET /api/v1/environments/{api_key}/document/.