helix-rest swapApis return result in json instead of by status code

[zpinto]

Issues

#2662

Description

Change canCompleteSwap and completeSwapIfPossible to return json with kv pair for result of check or attempt to complete swap.

Tests

• Add to updateInstance test

Changes that Break Backward Compatibility (Optional)

NA

Documentation (Optional)

NA

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]

Failed CI because of recurring flaky test: #2693

[zpinto]

This PR is ready to be merged.

Final Commit Message:

Change canCompleteSwap and completeSwapIfPossible to return json with kv pair for result of check or attempt to complete swap.