added isnotnull condition support

[ThyTran1402]

Description

Adds support for field IS [NOT] NULL as a predicate syntax in PPL, as a synonym for the existing isnull(field) / isnotnull(field) functions.
Previously, users coming from a SQL background would naturally write where field is not null in PPL, only to receive a parse error with no guidance.

Related Issues

Resolves #5262

Check List

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

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[github-actions]

Failed to generate code suggestions for PR

[github-actions]

Failed to generate code suggestions for PR

[ThyTran1402]

Do you know why it did not pass the tests 😢 ? @Swiddis

[Swiddis]

Unit tests are failing due to spotless violations, it says how to fix them here: https://github.com/opensearch-project/sql/actions/runs/23671443599/job/68965646433?pr=5278#step:6:9358

Doctests are failing because the cluster is returning incorrect results: https://github.com/opensearch-project/sql/actions/runs/23671443599/job/68965646435?pr=5278#step:6:661

Are you running these locally before pushing? Integ tests are slow locally, but at least ./gradlew spotlessApply spotlessCheck test doctest should get you most of the way there. You can always check the test action to see exactly what each CI check is doing. I also recommend setting up pre-commit, which will connect with our pre-commit steps to auto-fix your formatting on every commit.

If you want to work on this interactively, I'm available on the opensearch slack