[RLlib] DQN Rainbow Learner with new API stack (and EnvRunner API).

[simonsays1980]

Why are these changes needed?

We are moving DQN Rainbow over to the new stack using RLModule API and EnvRunner API. This PR introduces the learners for DQN Rainbow on the new stack.

Related issue number

Closes #37777

Checks

• [x] I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• [x] 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.
• [x] 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
  • [x] Unit tests
  • [ ] Release tests
  • [ ] This PR is not tested :(