Add ops flag to control partition manager

What does this MR do and why?

Add ops flag to control partition manager.

To be used during the upcoming PG12 to PG14 upgrade, to prevent partition manager from creating new tables in the gitlab_dynamic schema during the upgrade when both PG12 and PG14 replicas are in the connection pools.

Relates to https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/23578

MR acceptance checklist

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