Allow _in property of custom checks to accept wildcard passed in header

Currently when setting up a custom rule in the permissions system using the `_in` property of a rule against a set of values passed in with a `X-Hasura-*` header, the header is always required for requests from the user.

For certain users you may wish to disregard the rule and allow all results to be returned. One way to do this might be to allow the request to include a wildcard to indicate all results should be allowed.

When setting up a custom rule in the permissions system using the `_in` property of the rule to check a header for a list of values, allow the `X-Hasura-*` header to include a wildcard character to allow all.