Add task to check decomposed table counts

What does this MR do and why?

Adds a task to show tables that should have zero rows if decomposed.

We may incorporate this into gdk doctor, but I think this is also suitable to run for production as well

Related issue: gitlab-development-kit#1640 (closed)

Database queries

This runs a COUNT WITH LIMIT, such as:

• SELECT COUNT(*) FROM (SELECT * FROM ci_builds LIMIT 1) AS count https://console.postgres.ai/gitlab/gitlab-production-ci/sessions/17100/commands/57792

Screenshots or screen recordings

How to set up and validate locally

1. Run bundle exec rake gitlab:db:decomposition:check_decomposed_tables

MR acceptance checklist

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