Create risk score calculation service

What does this MR do and why?

This MR introduces the new Vulnerabilities::Findings::RiskScoreCalculationService. This service takes in one or more vulnerability ids and updates risk scores for the corresponding vulnerability_occurrences records. This is done through an upsert_all operation so records are either created or updated. This service is called in places where severity changes or a vulnerability transitions to an active state.

Risk score formula - https://gitlab.com/gitlab-org/secure/vulnerability-research/pocs/totalriskscore/-/merge_requests/1/diffs

References

Relates to - #569693 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

Updation Cases -

1. Go to vulnerabilities dashboard
2. Select one or more vulnerabilities and change severity.
3. Go to rails console and find the Vulnerabilities::FindingRiskScore records as described below and verify the risk scores based on the formula.

v = Vulnerability.find(id)
Vulnerabilities::FindingRiskScore.find(v.finding_id)

4. Do the same for state updates. Note - Only on transition to detected and confirmed state we calculate the scores, since we only care about active vulnerabilities.

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