Refactor CSV importers classes

What does this MR do and why?

Foundational refactoring to allow importing of work items in CSV format.

Introduces a new ImportCsv::BaseService which holds common logic related to importing CSV and creating the respective objects. It creates the flexibility required to introduce WorkItems::ImportCsvService allowing to import work items widgets data using the extra_create_service_params method which should be done in a follow-up.

related to Work Items: Restructure CSV export/import classes (#379081 - closed)
unblocks Work Items: Import basic data (#379153 - closed)

How to set up and validate locally

No user-facing change was introduced importing issues or legacy requirements objects using CSV should work as before.

MR acceptance checklist

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