Support an auth filter

This would then allow `countFoo` to work like `listFoo` with the auth filter. This basically just involves applying this logic:
```vtl
#if( !$util.isNullOrEmpty($ctx.stash.authFilter) )
  #set( $filter = $ctx.stash.authFilter )
  #if( !$util.isNullOrEmpty($args.filter) )
    #set( $filter = {
  "and":   [$filter, $args.filter]
} )
  #end
``` 