Introspection queries accept variables, but we need to make sure to
also touch the variables that we ignore, so that an introspection
query is marked not reusable if we are not able to build a correct
query plan for it.
A better solution here would be to deal with such unused variables
correctly, so that more introspection queries become reusable.
An even better solution would be to type-safely track *how* to reuse
which variables, rather than to split the reusage marking from the
planning.
Co-authored-by: Tirumarai Selvan <tiru@hasura.io>
