Add support for severity overrides in pipeline security finding type

What does this MR do and why?

Add support for severity overrides in pipeline security finding type, using a similar lazy loading approach introduced for vulnerability in Resolve "Add Severity Overrides to Vulnerabilit... (!176657 - merged) • Miki Amos • 17.9.

Relevant issue

Add severity overrides data to pipeline securit... (#511231 - closed) • Gal Katz • 17.9

How to set up and validate locally

Enable the FF:

pry(main)> Feature.enable(:vulnerability_severity_override))

Change severity of a pipeline security finding using this mutation:

mutation SecurityFindingSeverityOverride($findingUuid: String!, $severity: VulnerabilitySeverity!) {
  securityFindingSeverityOverride(input: {
    uuid: $findingUuid,
    severity: $severity
  }) {
    errors
    securityFinding {
      title
      severity
    }
  }
}

Use this query to get the finding severity override:

query getSecurityReportFinding($projectFullPath: ID!, $pipelineIid: ID!, $findingUuid: String!) {
  project(fullPath: $projectFullPath) {
    pipeline(iid: $pipelineIid) {
      securityReportFinding(uuid: $findingUuid) {
        severity
        severityOverrides(last: 1) {
          nodes {
            originalSeverity
            newSeverity
            createdAt
            author {
              name
            }
          }
        }
      }
    }
  }
}