cta-dcache: rename module to generic frontend-grpc

CMakeLists.txt

cta-dcache/README.md

cta.spec.in

frontend-grpc/CMakeLists.txt

frontend-grpc/FrontendGRpcSvc.cpp

frontend-grpc/FrontendGRpcSvc.h

frontend-grpc/Main.cpp

frontend-grpc/README.md

frontend-grpc/cta-frontend-grpc.1cta

frontend-grpc/cta-frontend-grpc.service

frontend-grpc/cta-frontend-grpc.sysconfig

frontend-grpc/grpc-proto/protobuf/cta_dcache.proto