Add E2E test of fork creation after storage change

What does this MR do?

Part of gitlab-org/quality/testcases#254 (closed)

Tests that if a fork is created and then the parent project is moved to a new gitaly storage/shard, another fork can be created after the move (see #38264 (closed)).

• Adds the ability to create a fork via the API, check import status (to confirm when the fork is complete), and change repository storage.
• Resource::Fork needed to be updated, which also required updating Resource::MergeRequestFromFork and the test that uses it so that that test can still fabricate a fork via the UI.

gitlab-qa MR with a new scenario to run this test: gitlab-qa!347 (merged)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers