Refactor #5025: Add env. vars. for graphql playground and introspection

[lehnenb]

Resolves #5025
Impact: minor
Type: refactor

Issue

In development, Apollo Server enables GraphQL Playground on the same URL as the GraphQL server itself (e.g. http://localhost:4000/graphql) and automatically serves the GUI to web browsers. When NODE_ENV is set to production, GraphQL Playground (as well as introspection) is disabled as a production best-practice.

Solution

Set environment variables with "envalid" in order to determine whether GraphQL Playground should be enabled or not in any environment.

Breaking changes

These changes are all pretty harmless, the only exception to that which has the capacity of causing an error is adding invalid boolean values to the newly added environment vars. (GRAPHQL_INTROSPECTION_ENABLED and GRAPHQL_PLAYGROUND_ENABLED).

FYI - Following are the accepted values for boolean environment variables in the "envalid" lib. : "0", "1", "true", "false", "t", "f"

Testing

1. Set the GRAPHQL_INTROSPECTION_ENABLED and GRAPHQL_PLAYGROUND_ENABLED;
2. Access GraphQL Playgrand on the same URL as the GraphQL server.

[aldeed]

Thanks for working on this! There are a couple things to fix, but after that should be good to merge.

[lehnenb]

@aldeed, thanks a million for reviewing my PR and for letting me contribute.
Please let me know if there's anything else I can help you guys with 👍

[aldeed]

Tested. Works!

[aldeed]

This is ready to merge, but I need to figure out how we can update the integration tests to continue passing. Envalid does not like the value of NODE_ENV when tests are running it seems.

[lehnenb]

That's Great @aldeed! Also, if you want, I can help you out with those failing tests too, just let me know 👍