[Feature Request] Make image feature extractor a parameter for CombinedExtractor

[manuel-delverme]

🚀 Feature

NatureCNN is hard coded for CombinedExtractor.

https://github.com/DLR-RM/stable-baselines3/blob/master/stable_baselines3/common/torch_layers.py#L258

Motivation

Implementing different networks requires rewriting or patching CombinedExtractor, it's a lot of code.

Pitch

I would be useful to have it as a default rather than hardcoded.

### Checklist

• [x] I have checked that there is no similar issue in the repo (required)

[Miffyli]

Thanks for the suggestion! I fully agree with this and am happy to see you already started a PR :)