Create Columns for standard and OSS cost factor admin

What does this MR do and why?

Issue: #363215 (closed)

In Gitlab::Ci::Minutes::CostFactor we find a cost factor based on a runner_matcher and a project.

• 1 standard cost factor(normally 1 but could vary based on runner)
  • We still have remnants of public/private cost factors which are stored on the ci_runner but these will be going away soon and will be consolidated into standard. We don't want to give discounts to public projects anymore.
• 0.5 (if the project has an open source plan. This won't currently vary based on the runner.)
• 0.008 (if the project is a open-source contribution. This won't currently vary based on runner.)

Here we introduce a new table to store these cost factors with a 1-to-1 relationship to the runner. These can then be set by admins.

Since we only bill for shared runners the table will only have 70ish rows.

How to set up and validate locally

1. Migrate the database

   rake db:migrate

2. View the new table

    psql -h <path_to_gdk>/postgresql -d gitlabhq_development_ci -p 5432
    select * from ci_cost_settings;
    /d+ ci_cost_settings

MR acceptance checklist

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