Support scopes

[phyrog]

Pundit supports the concept of scopes (basically filters for lists and list-like data).

We could support them with a scope instrumenter receiving a lambda, that in turn receives the usual root, arguments, context arguments.

We could then combine the scope instrumenter with the resource instrumenter like this:

resource ->(orgUnit, _args, _ctx) { orgUnit.repositories }
scope ->(repos, _args, _ctx) { RepositoryPolicy::Scope.new(current_user, repos).resolve }
resolve ->(filtered_repos, _args, _ctx) { filtered_repos }

or similar (with less boilerplate), but the gist is resource (-> authorize) -> scope -> resolve.