Restrict columns which are available to WHERE clause

I have the following DB table:
```
users:
- id
- first_name
- last_name
- email
- phone
- address
```

On this table, I only have indexes on the following columns: `id`, `first_name`, `last_name`, `email`.
The `phone` and `address` columns are NOT indexed.

Is it possible to restrict the set of allowed columns in a graphql WHERE clause to only the indexed columns? Further than that, is it possible to restrict the set of allowed operators at the column level?

If I do not have a database index on a column, or have specific kinds of indices, certain graphql queries would have poor performance, so I want to restrict the set of allowed WHERE queries to only those that would be performant.

Start by reviewing how Hasura defines GraphQL WHERE-clause fields and operators for the database table described in the issue. Determine whether indexed columns and allowed operators can be restricted at column level; done means the supported configuration and its performance-related limits are documented or implemented with coverage for the requested restrictions.

Written by the indexing model from the issue text.