Refactor logic for importing basic work item data from CSV

What does this MR do and why?

Imports basic work item data via CSV.

Related to #379153 (closed) - the first of multiple (see issue for MRs)

Depends on !111935 (merged).

How to set up and validate locally

No user-facing changes. In console:

project = Project.find(1)
user = User.find(1) # user with sufficient access to project
Feature.enable(:import_work_items_csv)
csv_io = FileUploader.new(project)
csv_io.store!(File.open("/path/to/work_items.csv"))
service = ::WorkItems::ImportCsvService(user, project, csv_io)
service.execute

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #379153 (closed)