Draft: Add `gitlab-database-load_balancing` gem

What does this MR do and why?

This is part of the #431748.

This is a potential change to be merged, that introduces and moves almost all code from gitlab/database/load_balancing into a new dedicated gem: gitlab-database-load_balancing.

This models all dependencies using dependency inversion principle, reducing the requirements of the load balancing code to a very small number of callbacks.

References

• Isolate `Gitlab::Database::LoadBalancing` code ... (#431748)
• Move `lib/gitlab/database` into its own Gems (&12005)
• GitLab Monorepo Gems (&10869)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.