input: Write a python function to convert hexadecimal to binary string.
  assert hex_to_binary('0x56') == '0b1010110'
  assert hex_to_binary('0x1A') == '0b11010'
  assert hex_to_binary('0xFF') == '0b11111111'
  assert hex_to_binary('0x0') == '0b0'
output: def hex_to_binary(hex_str):
      return bin(int(hex_str, 16))