Filter by identifier name graphql api

What does this MR do and why?

Vulnerabilities GraphQL API to filter by identifier_name on the project level field alone.

It is intended to be used by the frontend in #452492 (closed)

The API arguent is behind FF vulnerability_filtering_by_identifier. The argument is ignored if the FF is not enabled.

Screenshots or screen recordings

Database

Query plans are available in !171463 (comment 2199663954)

How to set up and validate locally

1. Import project https://gitlab.com/gitlab-org/govern/threat-insights-demos/verification-projects/verify-owasp-top-10-grouping into local. After import, go to Build -> pipelines and start a new pipeline this will feed the vulnerabilities data.

2. In rails console enable FF Feature.enable(:vulnerability_filtering_by_identifier)

3. In GraphIQL run the below equivalent query for the imported project

{
  project(fullPath: "bala-test-group/verify-filter-by-identifier") {
    vulnerabilities(identifierName: "CVE-2019-10086") {
      nodes {
        id
        identifiers {
          name
        }
      }
    }
  }
}

4. It should display only the filtered vulnerability with the identifier name as shown in the screenshot above.

Related to #432419 (closed)