RPC: Implement storage_stat_request (#800)

* RPC: Update protobuf sources
* RPC: Implement storage_stat_request
* RPC: Test storage_stat_request
* FuriRecord: fix use after free in destroy method.
* Furi: refactor PubSub and it's usage. Fix allocation in RPC.
* FuriCore: fix memory leak in pubsub
* FuriCore: update unsubscribe method signature in pubsub, make subscription structure lighter.
* FuriCore: remove dead code

Co-authored-by: Aleksandr Kutuzov <alleteam@gmail.com>

applications/rpc/rpc_i.h

@@ -1,5 +1,6 @@
 #pragma once
 #include "rpc.h"
+#include "storage/filesystem-api-defines.h"
 #include <pb.h>
 #include <pb_decode.h>
 #include <pb_encode.h>
@@ -29,3 +30,5 @@ void rpc_system_gui_free(void* ctx);
 
 void rpc_print_message(const PB_Main* message);
 void rpc_cli_command_start_session(Cli* cli, string_t args, void* context);
+
+PB_CommandStatus rpc_system_storage_get_error(FS_Error fs_error);