Draft: Use MultiStore in Redis::Sessions if configured

What does this MR do and why?

Implements https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14575

It is built on top of the !73660 (merged) (blocked by it), so the MR diff currently includes it too. To see a particular change, refer to the latest commit (I will keep all new changes squashed).

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.

Related to https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14575