Add `gitlab:db:lock_writes` a trigger to prevent writes due to gitlab_schema

What does this MR do and why?

Proof of concept usage of triggers to prevent writes outside of schema. Useful in context of the gitlab-com/gl-infra&693 (comment 879857950).

This does iterate all known tables on each database and creates or drops a trigger to prevent new INSERTs, UPDATEs, DELETEs or TRUNCATEs.

Related Issue: #361603 (closed)

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