Allow using callable objects as scopes

[palkan]

Context

Based on #192.

Let's make it possible using callable objects to extract scope implementations:

class ProjectPolicy < ApplicationPolicy
  relation_scope AuthorizedProjects

  # similar to
  relation_scope do |relation|
     AuthorizedProjects.call(relation)
  end
end

TODO

Let's update the .scope_for method to handle both blocks and callable objects, so, the new method definition would look like

def scope_for(type, name = :default, callable = nil, &block)
end

[palkan]

Closed by #263