Support for custom entities and eck

I've got some custom entities, created with ECK that I need to render through display suite. I've looked through the existing issues: #1839254: Entity Construction Kit compatibility for ds_views and #1864882: Fix base table and base id support in views (entities & fields), but that didn't get me very far, so I had to come up with another solution.

I'm attaching a patch that exposes a hook allowing modules to register their own combinations of base table, id field, pre_render function, and render function so that any entity can be exposed to the display suite views entity handler.

This patch also includes a ds_views_eck submodule that exposes all eck entities automatically.