Converting lazy abstract map to HashMap in ObjectFlatteners

[pranavbhole]

Fixes #XXXX.

Description

Removing the Lazy abstract map in ObjectFlatteners#flatten and making it as materialized HashMap with required fields from flattenspecs. Added tests to make sure that null value is accepted in the HashMap.

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.

[github-actions]

This pull request has been marked as stale due to 60 days of inactivity.
It will be closed in 4 weeks if no further activity occurs. If you think
that's incorrect or this pull request should instead be reviewed, please simply
write any comment. Even if closed, you can still revive the PR at any time or
discuss it on the dev@druid.apache.org list.
Thank you for your contributions.

[github-actions]

This pull request/issue has been closed due to lack of activity. If you think that
is incorrect, or the pull request requires review, you can revive the PR at any time.