Load MR Metadata when the MR Notes (Overview) app starts

What does this MR do?

For #300749 (closed)

1. Adds a mutation and action to ingest MR Metadata (which comes from the diffs_metadata.json endpoint)
2. Actually makes that network request in the app startup

It may be helpful to review this MR commit-by-commit, as each commit is atomic and quite small.

MR	Description
!56575 (merged)	This MR ingests the required endpoint into the MR Notes app.
We're here 👉🏻	This MR actually loads the diffs_metadata.json data and places it into the state for the MR Notes app
!57419 (merged)	This MR switches the Diffs app to use the MR Notes app location to fill the placeholder data
!58136 (merged)	This MR makes changes to enable getting the Diff file information into the Notes app on the Overview tab

Screenshots (strongly suggested)

N/A, no UI or UX changes in this MR.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • [-] I have included a changelog entry.
  • I have not included a changelog entry because there are no user-facing, non-code changes.
• [-] 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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