Configurable session cookie token prefix

What does this MR do and why?

Configurable session cookie token prefix

Addressing: #439945 (closed)

More context on why this is needed?

https://docs.gitlab.com/ee/architecture/blueprints/cells/iterations/cells-1.0.html#proposal

MR acceptance checklist

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Important note(s)

1. Changing the prefix doesn't the existing session cookies values. Only the new ones are affected. That's why in the validation steps we need to clear the cookies first.

How to set up and validate locally

1. When the config file exists and has the config

1. Create the file config/session_store.yml with this content

development:
  session_cookie_token_prefix: "random_prefix_"

1. Restart the Rails app gdk restart rails-web
2. Clear the cookies in the browser Right click -> Inspect -> Application (Tab) -> Cookies -> Select them all and delete them
3. After you refresh the browser you should see the the new gitlab session cookie value prefixed with the configured prefix value.

2. When the config file exists but the prefix is not there

1. Create the file config/session_store.yml with this content

development:
  something_else: "random_prefix_"

1. Restart the Rails app gdk restart rails-web
2. Clear the cookies in the browser Right click -> Inspect -> Application (Tab) -> Cookies -> Select them all and delete them
3. After you refresh the browser you should see the the new gitlab session cookie value not prefixed with anything. It should have the old 32 random characters and digits.

3. When the config doesn't exist

The same validation steps as the previous section (2)

Related to #439945 (closed)