feat: Add a health check endpoint.

[cullylarson]

Most of the hosting services we use (Render and Flightcontrol) require a health check endpoint. This PR adds such an endpoint that also includes some helpful info.

Checklist

• Requires dependency update?
• Generating a new app works

[kgajera]

Should we refactor this to use the config changes from #312?

[cullylarson]

Should we refactor this to use the config changes from #312?

@kgajera The only value from config that I've used in the health endpoint on other projects is stage. Most of the rest of the ones included here aren't directly referenced in config. E.g. NODE_ENV and APP_ENV are consolidated into stage. So it's helpful to see the values directly.