Explain plan for custom insert syntax

[LakshSingla]

Follow up to #12163.

Description

This PR adds the capability of running EXPLAIN PLAN FOR queries for statements with syntax having PARTITIONED BY and CLUSTERED BY clauses. Also, as a result, it disallows parsing insert statements that do not have a PARTITIONED BY.

explain.ftl encapsulates the logic to do so.

Code cleanup is also done since after this change, there is no need to handle SqlInsert and DruidSqlInsert separately in DruidPlanner's ParsedNodes.

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Key changed/added classes in this PR

• explain.ftl

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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.
• 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.