Reference x-hasura-user-id in query/mutation where clause

### Is your proposal related to a problem?

Column presets work great for inserts, but if I want to filter a query or mutation based on the user making the request, I don't want to expose the userId as a parameter/argument in my query/mutation. I can limit this with permissions, but it would be cleaner to be able to not expose the userId as a parameter.

### Describe the solution you'd like

When designing a mutation or query, it would be cool to be able to use something like `$$x-hasura-user-id` as a value, and have the server dynamically replace it with the `x-hasura-user-id` value. This way, we can reference the user who is making the request within where clauses without opening up a userid argument to arbitrary values.

### Describe alternatives you've considered

Alternatives to this issue would be to create an action that passes through my api so I can grab the user id, or limiting access to other rows with permissions.

Actions require creating more boilerplate code, and permissions sort of punt the problem.

### If the feature is approved, would you be willing to submit a PR?

Perhaps if I have the time.

Start by tracing how GraphQL query and mutation where clauses are handled and how x-hasura-user-id is made available during request processing. Compare the requested behavior with column presets and permissions, both mentioned in the issue, and identify tests covering where-clause values. Done means $$x-hasura-user-id works in both query and mutation where clauses without exposing a userId argument.

Written by the indexing model from the issue text.