[Core] Add demands info to GCS debug state

[rkooo567]

Why are these changes needed?

Example:

GcsAutoscalerStateManager: 
- last_seen_autoscaler_state_version_: 0
- last_cluster_resource_state_version_: 18
- pending demands:
	{CPU: 4, } * 88

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 :(

[rkooo567]

@jjyao comment addressed can we merge this before next release?

[jjyao]

@rkooo567 if we merge today, it should go out with the next release.