Encode the value for the attribute.

def Encode(self, attribute, value):
    """Encode the value for the attribute."""
    required_type = self._attribute_types.get(attribute, "bytes")

    if required_type == "integer":
      return rdf_structs.SignedVarintEncode(int(value))
    elif required_type == "unsigned_integer":
      return rdf_structs.VarintEncode(int(value))
    elif hasattr(value, "SerializeToString"):
      return value.SerializeToString()
    elif isinstance(value, bytes):
      return value
    elif isinstance(value, Text):
      return value.encode("utf-8")
    else:
      return str(value).encode("utf-8")