add GridWorlds environments

[Sid-Bhatia-0]

1. Allow incorporating environments from the GridWorlds package.
2. Add JuliaRL_BasicDQN_EmptyRoom experiment. This example also demonstrates the usage of RLEnv wrappers like ActionTransformedEnv, StateOverriddenEnv, RewardOverriddenEnv, and MaxTimeoutEnv.
3. Add test run in runtests.jl.

I think this experiment sufficiently demonstrates the basic usage of GridWorlds environments. I'm not adding too many experiments since this would just clutter RLZoo and overload the test suite.

[findmyway]

Thanks, looks great!

Could you also update the README.md?