Add a new endpoint: /features
The features are already provided under /first-rows, but some clients, like Autotrain (cc @SBrandeis), are interested in having access to the features, without the first rows.

Add a new endpoint: /features: The features are already provided under /first-rows, but some clients, like Autotrain (cc @SBrandeis), are interested in having access to the features, without the first rows.
