Refactor frontend-grpc to use frontend/common...

Problem to solve

Refactor frontend-grpc to use frontend/common in the drive to streamline the frontend and make it ready to only use gRPC to communicate with eos/xroot and dCache.

Stakeholders

CTA team, improving future maintenance.

Proposal

• Move code from frontend-grpc to frontend/grpc
• Refactor the configuration and initialization code to use the new FrontendService class
• Refactor the physics workflows to use the WorkflowEvent class, with shared-secret authentication (using gRPC tokens)