feat: expose state and key as attributes in DMEnvWrapper

[clement-bonnet]

Is your feature request related to a problem? Please describe

When working with a dm_env.Environment version of a Jumanji environment using the JumanjiToDMEnvWrapper, one may need to get/set the state and key of the environment e.g. to allow planning and "restart" the environment to its previous state.

Describe the solution you'd like

Right now, this is possible by calling wrapped_env._state and wrapped_env._state which should not be allowed since key and state are private attributes.
A solution would be to properly expose them as properties and to implement setters for these properties (in the common style).

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Checklist:

• Expose state and key of JumanjiToDMEnvWrapper as properties
• Implement setters for these properties
• Test that one can instantiate a Jumanji environment (e.g. jumanji.make("Snake-6x6-v0")), wrap it with JumanjiToDMEnvWrapper and then get and set the corresponding state and key attributes without leading underscores