Add base class for wrapping another adapter

[bkeepers]

This adds Flipper::Adapters::Wrapper, which can be used as a base class for an adapter that just wraps another adapter. By default, all methods delegate to the wrapped adapter. Implement #wrap to customize the behavior of all delegated methods, or override individual methods as needed.

class MyNewAdapter < Flipper::Adapters::Wrapper
  # Override a speciific method to change its behavior
  def enable(feature, gate, resource)
    # custom behavior
    super # invoke wrapped adapter
  end

  # Override this method to customize the behavior of all delegated methods, and just yield to
  # the block to call the wrapped adapter.
  def wrap(method, *args, **kwargs)
     if some_condition?(method)
       yield # call original adapter method
    else
       # do something else
    end
  end
end

And then you can configure Flipper to use the new adapter:

Flipper.configure do |config|
  config.use MyNewAdapter
end

[jnunemaker]

💯 on the branch name.

Merging because I need this in my cache branch.