Use integers as argument types of remote functions when registering

[bontric]

We currently use default values (e.g. "" for string) in register requests to specify the type of an argument of a remote function.
To improve compatibility, argument types should be represented by integers matching the types already in use (msg_object_type in sb-rpc.h):

typedef enum {
  OBJECT_TYPE_NIL,
  OBJECT_TYPE_INT,
  OBJECT_TYPE_UINT,
  OBJECT_TYPE_BOOL,
  OBJECT_TYPE_FLOAT,
  OBJECT_TYPE_STR,
  OBJECT_TYPE_BIN,
  OBJECT_TYPE_ARRAY,
} message_object_type;