Disable admin functionalilty if HASURA_GRAPHQL_ADMIN_SECRET is not set

### Is your proposal related to a problem?
To secure the GraphQL endpoint, I want to disable admin access.

### Describe the solution you'd like
By not setting the admin secret, admin functionality should be disabled.

### Describe alternatives you've considered
As a workaround, I'm removing the `x-hasura-admin-secret` header on my NGINX ingress controller if it's present, which breaks admin access on purpose.

For those interested:
```
nginx.ingress.kubernetes.io/configuration-snippet: |
more_set_input_headers "x-hasura-admin-secret: ";
```

### If the feature is approved, would you be willing to submit a PR?
I do not have enough understanding of the codebase to make a proper PR.

Start by locating the code that reads HASURA_GRAPHQL_ADMIN_SECRET and controls admin authorization, then trace how requests with the x-hasura-admin-secret header are handled. Done means leaving the secret unset disables admin functionality while ordinary GraphQL access remains available; add or update the relevant tests once their location is identified.

Written by the indexing model from the issue text.