sql: add support for statement-level triggers

[DrewKimball]

This issue tracks adding support for statement level BEFORE and AFTER triggers, which execute the trigger function exactly once. Just as for row-level triggers, statement-level BEFORE triggers execute the function before the mutation operation, while AFTER triggers execute the function after the mutation and its checks and cascades.

Jira issue: CRDB-37296

Jira issue: CRDB-39862