Vue merge request list app backend does not provide list of negatable arguments

The Issues list provides a list of negatable search filters, which is used to inform the GraphQL about what might appear as a not[] filter.

The merge request backend does not provide this list, although the resolver does indicate that some filters - like milestones - can be negated.

In order to implement the != filter for any token on the Vue merge request list, we'll need to be able to update the GraphQL with the $not property, which will need to use a list of negatable filters like the Issues list's NegatedIssueFilterInput.