Add a TaskReport for "kill" tasks

[kfaraz]

Changes

• Add KillTaskReport that contains stats for numSegmentsKilled, numBatchesProcessed, numSegmentsMarkedAsUnused
• Fix bug where exception message had no formatter but was was still being passed some args.
• Add some comments regarding deprecation of markAsUnused flag.

Release note

Kill tasks that delete unused segments now publish a task report containing kill stats such as numSegmentsKilled, numBatchesProcessed, numSegmentsMarkedAsUnused.

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[kfaraz]

Thanks for the review, @zachjsh ! I have updated the PR based on your feedback.

[zachjsh]

LGTM 🚀