[WiP] POC implementation of get_render_template

A possible solution for #3225

`get_render_template` must not be defined in CMSPlugin so we can statically check if it's implemented or not, thus the use of the *private* `_get_render_template` as method used by the API
Naming is temporary.

Docs and tests needed
`render_template` in the plugin model is actually a valid implementation in the original code (probably a legacy of past code as it's not documented as of now).
This simply preserves the current implementation.