GraphQL: Health check for GraphQL

Super low priority feat request, but if it's easy to add, it'd be nice during the early stages of a project to have a way to see if your GraphQL client is getting a response from your Hasura endpoint without having to worry about auth.

`/healthz` REST endpoint exists for general health check but I'd love to be able to test a GraphQL connection with something that completely bypasses all auth like

```gql
query HealthCheck {
healthz {
status
response_time
}
}
```

Start by reading the existing /healthz REST endpoint and the GraphQL API handling for authentication. Determine how a healthz query could return status and response_time while bypassing auth, then add coverage showing that the query works without credentials.

Written by the indexing model from the issue text.