[gRPC] Moves gRPC protobuf definition to a new submodule

The gRPC protobuf will be shared by EOS, CTA and CERNBox, so it needs to
go in its own submodule.

.gitmodules

@@ -2,3 +2,6 @@
 	path = xrootd-ssi-protobuf-interface
    url = https://:@gitlab.cern.ch:8443/eos/xrootd-ssi-protobuf-interface.git
 #   branch = stable
+[submodule "migration/grpc-proto"]
+	path = migration/grpc-proto
+	url = https://:@gitlab.cern.ch:8443/eos/grpc-proto.git

eos_cta/CMakeLists.txt

@@ -23,7 +23,7 @@ include_directories(${PROTOBUF3_INCLUDE_DIRS})
 # Select protobuf files
 set(PROTOBUF_EOS_CTA_DIR ${XRD_SSI_PB_DIR}/eos_cta/protobuf)
 file(GLOB ProtoFilesEosCta "${PROTOBUF_EOS_CTA_DIR}/*.proto")
-set(PROTOBUF_MIGRATION_DIR ${XRD_SSI_PB_DIR}/eos_cta/protobuf_grpc)
+set(PROTOBUF_MIGRATION_DIR ${PROJECT_SOURCE_DIR}/migration/grpc-proto/protobuf)
 file(GLOB ProtoFilesMigration "${PROTOBUF_MIGRATION_DIR}/*.proto")
 
 # Compile protobufs

xrootd-ssi-protobuf-interface @ 24594c24

-Subproject commit 5e5cb0b302c4225d8e47c2b788a1d91ce4fa57f5
+Subproject commit 24594c24566684a4da6bbb133c308260502ec625