Allow partitionAssignment API in maintenance mode

[GrantPSpencer]

Issues

Currently we do not return a partitionAssignment result if the cluster is in maintenance mode.

Allowing this would enable customers to make a series of changes to their cluster while in maintenance and then query the partitionAssignment one single time to get the expected assignments, without each change to the cluster triggering a rebalance.

Description

Allow partitionAssignment API to calculate an assignment while the cluster is in maintenance mode. This also includes a minor change to WAGED rebalancer in BestPossibleCalcStage, as computeResourceBestPossibleStateWithWagedRebalancer currently returns an empty map if the cluster is in maintenance mode.

Tests

• The following tests are written for this issue:

testComputePartitionAssignmentMaintenanceMode

Changes that Break Backward Compatibility (Optional)

• My PR contains changes that break backward compatibility or previous assumptions for certain methods or API. They include:

N/A

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Code Quality

• My diff has been formatted using helix-style.xml
  (helix-style-intellij.xml if IntelliJ IDE is used)

[zpinto]

Overall LGTM 👍
Some minor formatting comments.

[GrantPSpencer]

Overall LGTM 👍 Some minor formatting comments.

Good eye, fixed these in recent commit. Thanks!

[GrantPSpencer]

@xyuanlu can you re-run PR CI? The failing test failed 2 weeks ago as well, may be flaky
#2731