plpgsql: support Cursor FOR loop

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Jira issue: CRDB-33964
Epic CRDB-49018

The issue provides no files, tests, entry points, implementation details, or acceptance criteria. Start by reviewing Jira issue CRDB-33964 and epic CRDB-49018, then identify the relevant PL/pgSQL and SQL execution areas and define what Cursor FOR loop support must cover before implementation.

Written by the indexing model from the issue text.