Eliminate N+1 queries for pipeline GraphQL endpoint

What does this MR do?

This MR removes N+1 queries from the project pipeline GraphQL endpoint.

• The metadata relation is the most common N+1 query.
• The downstream_pipeline relation is for bridge jobs, not so common.

In this MR, the structure of preloading is also changed. (I have a plan to unify the preloading mechanism for commit statuses)

Related to #330707 (closed)

detailed comment: #330707 (comment 658019136)

previous work: !64700 (merged)

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

1. Run the development env with RAILS_PROFILE=true.
2. Create a pipeline with some jobs.
3. Open GraphiQL http://gdk.test:3000/-/graphql-explorer.
4. Trace the logs tail -f log/development.log in gitlab.
5. Run the query with variables;

query getPipelineDetails($projectPath: ID!, $iid: ID!) {
  project(fullPath: $projectPath) {
    pipeline(iid: $iid) {
      stages {
        nodes {
          groups {
            nodes {
              jobs {
                nodes {
                  status: detailedStatus {
                    detailsPath
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

6. See the logs.

Metrics

This will help us to check later:

75th and 99th percentiles of duration and db count: https://log.gprd.gitlab.net/goto/b08207d25d2259903d5140b938f612e7

Last 7 days:

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team