Boolean function in PPL should be case insensitive

[LantaoJin]

Description

isnotnull() works but ISNOTNULL() doesn't work in PPL.

Issues Resolved

Resolve https://github.com/opensearch-project/sql/issues/2431

Check List

• [x] New functionality includes testing.
  • [x] All tests pass, including unit test, integration test and doctest
• ~~[ ] New functionality has been documented.~~
  • ~~[ ] New functionality has javadoc added~~
  • ~~[ ] New functionality has user manual doc added~~
• [x] Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license. For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[LantaoJin]

Thanks for your reviewing @penghuo @dai-chen @vamsi-amazon , will this PR be merged automatically? And backport PR will be triggered automatically either? Any action should I do next?

[vamsimanohar]

@LantaoJin Could you confirm if the build issues are not because of your commit. I will merge all the approved PRs.

[LantaoJin]

@LantaoJin Could you confirm if the build issues are not because of your commit. I will merge all the approved PRs.

None of current build issues caused by this code changes.

[penghuo]

could u merge latest change from main and re-run test.