Add epic to work item migration e2e spec

What does this MR do and why?

Adds a temporary e2e spec for epic to work items migration sync. This spec will only run against staging in a separate scheduled pipeline as per request since staging has ever-changing feature flags set up for this sync to work. Once the migration is complete, we can alter the code to replace legacy epics in tests with work item epics.

This MR also fixes how we return graphql responses and updates the resources that use them. Flattening the hash would rewrite values of keys with the same name. For example a graphql response of:

{ id: 7, author: { id: 5 }, title: "This is a title" } Would flatten to: { id: 5, title: "This is a title" }

With the changes in this MR, we now keep the structure of the graphql resource.

gitlab-org/quality/quality-engineering/team-tasks#2651 (closed)

MR acceptance checklist

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How to set up and validate locally

To test locally against staging, from gitlab/qa run:

EPIC_SYNC_TEST=true GITLAB_QA_USER_AGENT=<found_in_1Password> GITLAB_USERNAME="gitlab-qa" GITLAB_PASSWORD=<found_in_1Password> bundle exec bin/qa Test::Instance::All https://staging.gitlab.com -- qa/specs/features/ee/api/2_plan/epics_to_work_items_sync_spec.rb