Make dropExisting flag for Compaction configurable and add warning documentations

[maytasm]

Make dropExisting flag for Compaction configurable and add warning documentations

Description

In PR #11025 I have introduced a new option for ingestion task to drop (mark unused) all existing segments that are contained by the interval in the ingestionSpec. However, it was pointed out after the PR was merged that this new option can cause temporary data unavailability for data within the specified interval of the ingestion task. Compaction task created manually and by auto compaction were hardcoded to always use the new dropExisting flag in PR #11025. This PR make dropExisting flag for Compaction task configurable and set the default behavior when dropExisting is not specified to not use the dropExisting functionality (which is the same behavior as before the PR #11025). This PR also adds documentation with warning to temporary data unavailability if dropExisting flag is used.

This PR does not contain the fix for this problem yet. The fix will come later.

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.
• 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.

[jihoonson]

@asdf2014 I removed the 0.21.0 milestone because this PR is a follow-up of #11025 which was merged into only master.

[loquisgon]

LGTM

[asdf2014]

@jihoonson Sure, thanks.

[maytasm]

@asdf2014
I don't think this PR needs the "Design Review" tag as it does not introduce any major architectural, backwards incompatible changes, or changes which have long-term maintenance consequences. The actual feature was introduced in #11025. This PR simply adds a feature flag to the feature introduce in #11025, and does not introduce or modify any feature. This PR is also not a breaking change.

[zachjsh]

LGTM