[Misc] Respect CTA gRPC frontend configuration options to block user and/or repack operations

Description

This commit ensures that the options

• cta.schedulerdb.enable_repack_requests on|off
• cta.schedulerdb.enable_user_requests on|off are respected on the CTA gRPC Frontend

Checklist

• Documentation reflects the changes made.
• Merge Request title is clear, concise, and suitable as a changelog entry. See our contributing docs

References

Closes #611 (closed)