If I’m not mistaken, this module doesn’t provide any classes, just type definitions - so not sure what exactly you’re expecting to see in the puppet classes page?
With type definition you get the possibility to define resources, so in this case the module " augeasproviders_grub" enables you to define resources like kernel_parameter, grub_config, grub_user and grub_menuentry.
To define this resources you will need to write your own puppet code or take a module which does this, but without a class structure around this module there is nothing Foreman can present to you in the frontend or your puppet as ENC.
The same problem exists if a module does not provide a class but a defined type which is nearly the same but written in Puppet syntax instead of Ruby.