feat: add image render endpoint

This endpoint adds a quick way for clients to request arbitrary image sizes based on the functionality available in the versatileimagefield package.

If we have more advanced use cases we could either take a look at integrating libvips [1] or delegate requests to a thumbor instance [2].

[1] https://github.com/libvips/pyvips [2] https://github.com/thumbor/thumbor