chore: separate atkey upload to a service in at_onboarding_flutter

[xavierchanth]

- What I did

• Extracted the file upload portion of the onboarding home screen to it's own service.
• Moved everything from the root of the package to src/ so we have better grasp over our public API.
• Added new export files, selectively exporting by context, and preserving the original exports.

- How I did it

- How to verify it

• Review commit by commit, as there are a lot of faux changes due to the file renames:

• Tested on macos / ios simulator

- Description for the changelog chore: separate atkey upload to a service in at_onboarding_flutter

[xavierchanth]

@gkc I originally planned to do a lot more, but I believe this service extraction is the minimum required changeset for us to make the planned improvements to the NoPorts desktop onboarding UX.

[xavierchanth]

Setting back to draft, need to make a few more changes