Prepare Jira users importer

What does this MR do?

It prepares a basic user importer service classes. The importer will be used during the Jira import initialization and the results will be returned in graphQL endpoint. The endpoint will be used by the FE which will display the users mapping form.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Preparation for #216145 (closed)