Add mvexpand PPL Command: Expand Multi-value Fields into Separate Rows

[srikanthpadakanti]

Description

This pull request introduces the new mvexpand Piped Processing Language (PPL) command to OpenSearch SQL. The mvexpand command enables expansion of multi-value fields—such as arrays—into separate rows, allowing users to easily normalize and analyze nested data. This supports advanced transformations in data pipelines and complements existing PPL commands.

Key highlights:

Implements AST node, logical and physical plan wiring for mvexpand.
Adds support for multi-value expansion for any array field in tabular results.
Includes testing and documentation for the new command.

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• [ X ] New functionality includes testing.
• [ X ] New functionality has been documented.
• [ X ] New functionality has javadoc added.
• [ X ] New functionality has a user manual doc added.
• [ X ] New PPL command checklist all confirmed.
• API changes companion pull request created.
• [ X ] Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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[penghuo]

Thanks for the PR.
Early feedback, Could u reference mvjoin support in PPL Caclite.

[srikanthpadakanti]

mvjoin support in PPL Caclite

Hello @penghuo Thank you for the message. But I do not see any files here