sql: add a new optional secondary logger for internal slow queries

[jseldess]

PR: cockroachdb/cockroach#52377

From release notes:

Added a new cluster setting sql.log.slow_query.internal which, when turned on in conjunction with the slow query log, causes slow internal queries to be logged to a slow internal query log at cockroach-sql-slow-internal-only.log. Internal queries are no longer logged to the slow query log. This new setting is opt-in. The default behavior is to not log slow internal queries. [#52377][#52377] {% comment %}doc{% endcomment %}