input: Write a function to replace all occurrences of a given character in a string with the provided replacement character. If the character to be replaced does not exist in the string, return the original string.
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Python is awesome", 'o', 'Z')=="PythZn is awesZme"
  assert replace_all_occurrences("Coding is fun", 'a', 'Y')=="Coding is fun"
  assert replace_all_occurrences("", 'x', 'Z')==""
  assert replace_all_occurrences("Hello", 'H', 'h')=="hello"
output: def replace_all_occurrences(s, char_to_replace, replacement_char):
      return s.replace(char_to_replace, replacement_char)