Setting "Description" and other meta data against a product

See also PS365-I-2220        

Basically, we should be able to explicitly set meta data against a product

For title and description and keywords (probably other common ones too), we should allow you to use the product name if nothing else is set, but we should offer these fields in product manager for groups and products allowing you to explicitly set them.

And, ideally also allow the customer to configure any other meta fields they want to set against products/groups and dynamically add these fields to the forms.

I realise this is chicken and egg with rewriting the product plugin, but to be honest we should just add the fields to product manager so customers can start populating it, add the capability to the CMS and product categories plugin and then add to product plugin when we rewrite it.