Support explicit transaction in compiler

[zhanglei1949]

We should at least support parsing statements

BEGIN TRANSACTION
BEGIN TRANSACTION READ ONLY
COMMIT
ROLLBACK

Those statement should not be mixed with other queries.
Maybe we should introduce more flags in ExecutionFlags: https://github.com/alibaba/neug/blob/main/include/neug/compiler/gopt/g_physical_analyzer.h#L43

[github-actions]

🤖 Auto-triage: linked as sub-issue of #86 — Refactor UpdateTransaction (confidence: high).

Reason: The issue is about adding explicit transaction control statements (BEGIN, COMMIT, ROLLBACK) to the compiler, which directly relates to the transaction layer and UpdateTransaction surface.

Auto-applied: labels → transaction

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