Import work items with work item type via CSV

What does this MR do and why?

Handle different Work Item types when importing Work Items via CSV.

This MR introduces the following behaviour:

• Work items with a work item type can be imported via CSV. The work item type is identified by the name.
• If a work item type - is not supported, we throw an error
• If a user cannot create the kind of work item in question, we throw an error.

Also, generally:

• a validate_structure! method that checks the headers and column separators before any other processing begins.
• a preprocess! method that can be overridden in subclasses to handle existence of associated objects, called in the work item importer.

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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

Related to #379153 (closed)