input: Write a python function to convert a hexadecimal string to a binary string.
  assert hex_to_binary("0xA2") == "10100010"
  assert hex_to_binary("0xFF") == "11111111"
  assert hex_to_binary("0x1") == "1"
  assert hex_to_binary("0x0") == "0"
output: def hex_to_binary(hex_string):
      return bin(int(hex_string, 16))[2:]