Presets for custom tracked function arguments

Today, it is possible to set custom presents on table fields, which is one of the best Hasura features. The same is true for remote GraphqQL schemas.

It would be great if this feature could be extended to custom-tracked functions.

For instance, if I defined the following function:

```sql
create or replace function search_foo(tenant_id int, keyword text)
returns "foos" as
$$
...
$$;
```

I would like to add a preset on the `tenant_id` argument for certain roles. The argument should disappear from the GraphQL node and be internally provided from session variables.