input: Write a python function to find all words in a given string that begin with a specified letter.
  assert all_words_beginning_with("Apple is awesome and amazing", 'a') == ['Apple', 'awesome', 'and', 'amazing']
  assert all_words_beginning_with("Bananas are best but blueberries are better", 'b') == ['Bananas', 'best', 'but', 'blueberries', 'better']
  assert all_words_beginning_with("Nothing starts with the letter z here", 'z') == []
output: def all_words_beginning_with(s, letter):
      return [word for word in s.split() if word.lower().startswith(letter.lower())]