Add configuration on CTA gRPC frontend to block user and/or repack operations

Situation

On issue #573 (closed) we have added a configuration to the CTA SSI frontend that can block user and/or repack operations.

This was a requirement to split the objectstore backend between repack and user instances, which left us with two different groups of CTA frontends, each one communicating with a different objectstore backend.

To make it work properly, #573 (closed) added the hability to configure each frontend to block operations of the wrong type.

---

This was added for the CTA SSI frontend, but was not made yet available on the gRPC frontent.

Task

Make these two CTA frontent configurations work on the CTA gRPC frontend:

• cta.schedulerdb.enable_repack_requests on|off
• cta.schedulerdb.enable_user_requests on|off

Ideally, this should be implemented in the new FrontendService class, intended to be shared between the two frontend types (work in progress #337 (closed)).
This way, the logic would be abstracted from the communication protocol and common for both.