Adds argument to set obs key to stable_baselines_export.py

[Ivan-267]

Adds the ability to change the obs key used when exporting to onnx.

Usage:

1 - In SB3 example script, find export_model_as_onnx method, and inside it set the obs_key, e.g.:

export_model_as_onnx(model, str(path_onnx), obs_keys=['obs2'])

(recommended to use a single key for now, as that's what inference supports for now)

2 - In sync node in GDRL plugin in Godot, find the _inference_process: method and set the correct key, e.g.:

			var action = model.run_inference(
				obs[agent_id]["obs2"], 1.0
			)

(this should also be adjustable in the future for each agent, but as we support only one key, it's currently only necessary for the camera sensor so the Python server can detect it as image obs and run needed processing, in other cases there's no advantage from renaming the obs key)

Note

If you don't set the key or modify anything, obs will be used by default as before. This is the default which all examples use.

Note

Onnx export/inference with the camera sensor is not guaranteed to work with this update. SB3/Rllib and other frameworks can do their own pre-processing which adds complexity to the implementation. However, this PR is still a required step toward it.