Add free user cap feature flag and availability check

What does this MR do and why?

• Adds feature flag for free user cap project see #349811
• Adds predicate method to encapsulate when user cap and free user cap are both checked.
• Performs a side-effect free check to see if a namespace has a free subscription - see explanation in !80839 (comment 851566461)

broken out from !80518 (merged)

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 #349811