Resolve "Create class for generating user data export file"

What does this MR do?

This is the first MR part of &4207 (closed). It introduces a new service to generate a CSV string which includes the current license information in the header, followed by a table of historical user counts. It will be generated by a self-managed instance customer to deliver to a GL salesperson for a clearer picture of their usage in order to process quarterly coterms or renewals. It won't really be consumed by the customer. We plan on encrypting this file in a future iteration.

Here's an example of the file: example.csv

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.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Relates to #235882 (closed)