[Feature Request] Env Wrapper for specifying a context length of past obs + actions

[RuiningLi]

It would be nice to have a wrapper under brax.envs.wrappers which takes an Env and a context length, and wraps the Env so that the obs space of the new Env contains previous L steps of observation and action.

[erikfrey]

That it indeed would be a useful wrapper! I've marked it as a good first issue for anyone who might want to roll up their sleeves and take a crack at it.