Implement lazy association loader

What does this MR do and why?

This MR introduces an abstraction to preload associated records of an Active Record model lazily on GraphQL. It makes it possible to sort, limit, and filter has_many associations without causing an N+1 query issue.

See [Draft] Use lazy association loader (!115241 - closed) for its usage.

Database review

The below queries are generated for a GraphQL query to load only the last state transition record for a list of vulnerabilities.

GraphQL query

query getVulnerabilities($project_fullpath: ID!) {
  project(fullPath: $project_fullpath) {
    vulnerabilities(first: 20) {
      nodes {
        stateTransitions(last: 1) {
          nodes {
            fromState
          }
        }
      }
    }
  }
} 

Here is an example query generated by the `TopNLoader` utility class

SELECT
    "vulnerability_state_transitions".*
FROM
    vulnerabilities
    JOIN LATERAL (
        SELECT
            "vulnerability_state_transitions".*
        FROM
            "vulnerability_state_transitions"
        WHERE (vulnerability_state_transitions.vulnerability_id = vulnerabilities.id)
    ORDER BY
        "vulnerability_state_transitions"."id" ASC
    LIMIT 2) AS vulnerability_state_transitions ON TRUE
WHERE
    "vulnerabilities"."id" IN (78324807, 78324806, 78324805, 77964096, 77896757, 77896756, 77896755, 77896754, 77625632, 77625631, 77625630, 77524835, 77524834, 77524833, 77498088, 77498087, 77498086, 77069619, 77069618, 77046383)

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/16812/commands/57213

Old query to load all the state transitions

SELECT
    "vulnerability_state_transitions".*
FROM
    "vulnerability_state_transitions"
WHERE
    "vulnerability_state_transitions"."vulnerability_id" IN (78324807, 78324806, 78324805, 77964096, 77896757, 77896756, 77896755, 77896754, 77625632, 77625631, 77625630, 77524835, 77524834, 77524833, 77498088, 77498087, 77498086, 77069619, 77069618, 77046383)

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/16938/commands/57551

Note that the latter one performs better but it loads all the state transitions from the database which could introduce memory bloat and serious performance issues.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.