Spike: Should Mailroom gemspec handle dependencies

Overview

Should the CNG install gem dependencies for Mailroom or should they be in the Mailroom gemspec?

Context

In review of Update dependency redis-client to v0.25.3 (!2645 - merged) • GitLab Dependency Bot • 18.6, @WarheadsSE and I had a discussion about test relevance and where the redis-client dependency should get tested.

We want to investigate whether the dependencies installed at https://gitlab.com/gitlab-org/build/CNG/-/blob/master/gitlab-mailroom/scripts/install-dependencies?ref_type=heads should instead become entries over at https://gitlab.com/gitlab-org/ruby/gems/gitlab-mail_room/-/blob/master/mail_room.gemspec?ref_type=heads

Deliverables

• Investigate how feasible it would be to move the various rubygems dependencies into the Gemspec for the mailroom gem.
• Check on the impact to Omnibus GitLab's builds.
• Make a decision, schedule the work to change the Gemspec or update documentation about why we did not choose to migrate the dependencies.