Add cell routing info to new runner tokens

What does this MR do and why?

This MR changes the logic that computes authentication tokens for new runners, to include routing info that can later be used by the router to decide to which cell to forward an unauthenticated API request to.

cell	old token	new token	short sha version
1	glrt-UfMt7-xxxxxx-ZJpuvsb	glrt-cell_1_UfMt7-xxxxxx-ZJpuvsb	UfMt7-xxx
1	UfMt7-xxxxxx-ZJpuvsb	cell_1_UfMt7-xxxxxx-ZJpuvsb	UfMt7-xxx
2	glrt-UfMt7-xxxxxx-ZJpuvsb	glrt-cell_2_UfMt7-xxxxxx-ZJpuvsb	UfMt7-xxx
2	UfMt7-xxxxxx-ZJpuvsb	cell_2_UfMt7-xxxxxx-ZJpuvsb	UfMt7-xxx

Closes #458419 (closed)

There is a matching MR on the GitLab Runner side to have it ignore the cell routing info when computing a short sha token: gitlab-runner!4746 (closed)

MR acceptance checklist

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Screenshots or screen recordings

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Before	After
This request would have gone to cell 1, which would not recognize the token:

How to set up and validate locally

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

1. Run the following command to provide a cell prefix to your GDK installation (from https://gitlab.com/gitlab-org/tenant-scale-group/pocs/routing/rules-router#step-1-gdk-1):

   gdk config set gitlab.rails.session_store.session_cookie_token_prefix cell_2_
   gdk reconfigure
   gdk restart

2. Create a new runner at http://gdk.test:3000/admin/runners/new

3. Confirm that the generated runner token contains the configured cell prefix.