Add the user update API endpoint to the Diffs app

What does this MR do?

For #284116 (closed)

In preparation for saving the user's preference when they change it from the Diffs app ⚙ dropdown menu we're bringing the endpoint to do so into the Diffs app.

There is no UX change here, we're just preparing for those changes.

MR	Description
!55033 (merged)	Exposes the endpoint to update the user settings
We're here 👉🏻	Update the Diffs app to ingest and store the endpoint that we will use to update the user preference for file-by-file in the future
!55931 (merged)	Update the Diffs app to persist the user's Single File preference to their account when they change it from the ⚙ dropdown menu

Screenshots (strongly suggested)

N/A, all backstage type work

Does this MR meet the acceptance criteria?

Conformity

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  • [-] I have included a changelog entry.
  • I have not included a changelog entry because this is developer-facing work only.
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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:

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