[Feature Request] GraphFeatureExtractor

### 🚀 Feature

Graph feature extractor for the new observation space that gymnassium provides.

### Motivation

I've been using a Graph2Vec aproximation using GraphTransformers and CLS tokens on a non-publishable custom environment. As Gymnassium has implemented the Graph observation space, I would like to code a Feature Extractor for that kind of observation spaces.

### Pitch

I want agents to use graph as observations

### Alternatives

The alternative is to create a custom feature extractor that handles a dictionary space of the matrixes and attributes of graphs, but when the graph is very big this is not very optimal. The final goal would be to work with batches of graphs and sparse tensors.

### Additional context

The only remaining piece is an official or publicly available gym environment to test this Feature Extractor. Does anyone know about this?

### Checklist

- [X] I have checked that there is no similar [issue](https://github.com/DLR-RM/stable-baselines3/issues) in the repo
- [X] If I'm requesting a new feature, I have proposed alternatives

No implementation files or tests are named. Start by reviewing the existing feature-extractor interfaces and how Gymnasium observation spaces are handled, then define support for graph observations, batched graphs, and sparse tensors. Done requires a concrete public environment or test fixture and agreed behavior for the extractor.

Written by the indexing model from the issue text.