Disable DELETE clause by defaut and add opensearch.sql.delete.enabled setting

[penghuo]

Description

1. By default, the DELETE clause is disabled in SQL engine.
2. Add the opensearch.sql.delete.enabled setting which could be used to enable the DELETE clause.
3. Update the docs, https://github.com/penghuo/os-sql/blob/pr-issue59/docs/user/admin/settings.rst#opensearch-sql-delete-enabled

Issues Resolved

#59

Check List

• New functionality includes testing.
  • 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
• Commits are signed per the DCO using --signoff

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[dai-chen]

Thanks for the changes!