[IO-1276] 3-step upload (#693)

* Initial WIP on item uploads

* Further skeletoning of upload endpoint

* Some implementation of all methods now in place, outline of testing.

* Linting fixes

* All methods implemented, layout helper pending.

* Linting

* WIP: Tests for build items

* WIP Tests

* Build items fully working

* Tests WIP

* WIP all but register endpoint tests

* Unit tests complete for 3-step upload

* Linting

* Tests for relevant parts of data items

* Formatting

* Linting

* Disable E501 in pyproject

* Fix actual ruff thing

* Refactor based on sync

* Discarded idea of returning Items

* Fixing some tests

* Test fixes

* Lint fixes

* Refactor to introduce UploadItem to make ID and other non present members unnecessary at upload

* Test fix, dogfooding done.

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Co-authored-by: Owen <[email protected]>