Add DoraPerformanceScore model

What does this MR do and why?

Adds new model to store denormalized DORA performance scores. This MR adds the model only. Actual data calculations will be done in a follow-up MR.

What is the anticipated growth for the new table over the next 3 months, 6 months, 1 year? What assumptions are these based on?

Feature is Ultimate only data is stored 1 row per month per project. So growth will be relatively small. number_of_ultimate_projects*12 per year.

How many reads and writes per hour would you expect this table to have in 3 months, 6 months, 1 year? Under what circumstances are rows updated? What assumptions are these based on?

Current month row will be updated on each DORA metric update. So on each successful deployment or issue close. Data for previous months will be updated very rarely.

Based on the anticipated data volume and access patterns, does the new table pose an availability risk to GitLab.com or self-managed instances? Does the proposed design scale to support the needs of GitLab.com and self-managed customers?

No. Yes.

MR acceptance checklist

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Related to #386844 (closed)