Fix get data requests

[guarani]

Description

The code was attempting to use a JSONDecoder to decode responses when the caller was asking for plain Data. The fix is to make a separate function for getting Data responses without any JSON decoding.

Related discussion: p1709846681819879/1709766129.730749-slack-C04PWEZSYFL

ℹ Please replace the above with a link to the issue this pull request addresses, as well as a summary of the implementation details.

Testing Details

Test a clean install using wordpress-mobile/WordPress-iOS#22792 and verify that a feature announcement is shown (called "Stats Refresh"). You must log in using your A8c account.

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[guarani]

👋 Hey @crazytonyli, it looks like the tests need to be updated since the perform() function now has a new overload) and I didn't get to that today. I can loop back later tonight or tomorrow to fix. Feel free to hold off reviewing, or just review the code changes and I can update the tests and merge tomorrow 🙇

[guarani]

Closed in favor of #746.