sql: support system/event triggers

[DrewKimball]

Postgres allows "event" triggers to be defined to execute a trigger function when certain schema changes happen within the database. See the Postgres docs here: https://www.postgresql.org/docs/16/event-trigger-definition.html

Oracle has a similar concept of "system" triggers, which can be associated with additional events, such as user login. See the Oracle docs here: https://docs.oracle.com/en/database/oracle/oracle-database/19/lnpls/plsql-triggers.html#GUID-FE23FCE8-DE36-41EF-80A9-6B4B49E80E5B

This issue tracks adding support for Postgres-style event triggers, possibly with some Oracle-like extensions.

Epic CRDB-347

Jira issue: CRDB-39864

Jira issue: CRDB-39864