[Backport 2.19-dev] Fix sub-fields accessing of generated structs (#4683)#4721

LantaoJin merged 2 commits intoopensearch-project:2.19-devopensearch-project/sql:2.19-devfrom

yuancu:backport/backport-4683-to-2.19-devyuancu/sql-plugin:backport/backport-4683-to-2.19-devCopy head branch name to clipboard