(#7013) Support 'when_rendering' and 'render_as' in actions.

These define the API used by folks writing actions that supports their
rendering hooks. 'when_rendering' defines a helper method on the interface,
which runs the users code in their expected context.

'render_as' just sets the default rendering format; by default this is
:for_humans.

Reviewed-By: Max Martin <max@puppetlabs.com>