Make numCorePartitions as 0 for tombstones

[abhishekrb19]

This PR changes the core partitions for tombstones to 0, so they can exist independently if they're deleted or marked as unused without affecting availability of other appended segments in the same co-partition space. This is particularly important in the following scenarios:

1. Concurrent replace and append to a datasource containing tombstones and data segments in the same time chunk (also, related coordinator duty: Clean up duty for non-overlapping eternity tombstones #15281)
2. If a user drops a tombstone segment and there are other data segments appended to it, the data segments should still be available

Release note

Tombstone segments now have 0 core partitions instead of 1. This means they can be dropped or removed independently without affecting availability of other appended segments in the same co-partition space. Note that removing tombstones with 1 core partition (prior to this change) that contain appended segments in the partition space can make the appended segments unavailable.

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Key changed/added classes in this PR

• TombstoneShardSpec.java
• IndexerSQLMetadataStorageCoordinatorTest.java

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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 unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• been tested in a test Druid cluster.

[AmatyaAvadhanula]

Thanks @abhishekrb19! LGTM

[kfaraz]

Conceptually, the change makes sense to me but taking another pass just to be sure that it doesn't break any existing assumption in the code. Also, there are a couple of test failures.

[abhishekrb19]

Thanks for the reviews!