[RLlib] Examples folder cleanup: Add example script for Env with protobuf observation space.

[sven1977]

Examples folder cleanup: Add example script for Env with protobuf observation space.

PR adds:

• Env class that uses a protobuf (binary string) observation space.
• ConnectorV2 piece that decodes the protobuf and translates it into a NN-readable tensor.
• Example script showing that CartPole w/ protobuf space can be learnt normally by PPO.

Why are these changes needed?

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[simonsays1980]

LGTM. Awesome example!