Refactor billable user finder for SSOT

What does this MR do and why?

Note No new behavior, refactor only

• moves billable counting logic to a finder as a general refactor that will enable this subsequent MR that will add limit and other improvement to the free user count - Improve billable users query (!104787 - merged)
• improves the testing of each user count method
• moves the counting queries to public space of the group model for use in the finder

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #383264