Add an "is_signed" attribute to the quant_ConstFakeQuant op

Some TensorFlow simulated quantize ops such as QuantizeAndDequantizeV2Op have
attribute for the sign of the quantization, so quant_ConstFakeQuant should be
able to represent it with the new attribute is added.

The method for converting these attributes to an QuantizedType is updated to
handle this new argument.

PiperOrigin-RevId: 258810290