Is your feature request related to a problem? Please describe.

To mitigate the risks of #70731, #70976 disabled having multiple subquery modifications on the same table in the same transaction and created the sql.multiple_modifications_of_table.enabled cluster setting to re-enable it.

Having it as a cluster setting feels too broad, however. A cluster admin may be forced to enable it for an important query if it is safe to run. However, subsequent new queries may hit the problems of #70731 without anyone noticing.

Describe the solution you'd like
It would be better if sql.multiple_modifications_of_table.enabled can be toggled as a session variable. This allows to locally reason about particular queries.

Jira issue: CRDB-13051