Provide a mechanism for storing a user id to a Rack session for active_record_store.

[jvlcek]

This is a sub-issue of issue Ability to track and revoke sessions #20378

Brief:

The ultimate goal is to be able to look up all current sessions stored in active_record_store for a given user id. This will require finding a mechanism for storing the user id as part of the Rack session for indexed lookup.

Details:

The backing store for a session is based on the Settings.server.session_store value and is interpreted in the session_store initializer code.

There are three different types:

• active_record_store ultimately ends up using the Session model
• memory_store is in-memory
• mem_cache_store ultimately uses Rack::Session::Dalli from the dalli gem.

In all cases, we use the underlying Rails session stores.

There will be a separate issue for each one of the three since each can be addressed separately, with separate PRs.

• As the subject implies, this issue will be used to track the active_record_store case.
• The memory_store work is tracked in Issue: #20464
• The mem_cache_store work is tracked in Issue: #20462

[gtanzillo]

Rolled into #20462